The harm would come from the fact that you’ve worn shoes all your life and your feet are adapted to that. Someone who’s never worn shoes is accustomed to the forces the foot experiences when bare. Shoes take some of that stress and the feet are “weaker”. You can build up to it just like any other exercise.
Sorry, no links to research, just going by experience and WAG.
Yup, I worked on tribes where nobody ever wore shoes. They could walk miles on pebbles without flinching. I could not match them in my wildest dreams, and I am someone very used to being barefoot/without arch support for modern standards.
Walking “naturally” on soft surfaces is less healthy if you injure you foot, which is certainly more likely while barefoot than shod. Additionally, if you experience foot pain (or leg or back pain) while walking barefoot and you don’t have an obvious injury, it’s probably because you are over pronating, which apparently 60-80% of the population does (link is a cached HTML page; original is PDF). Additionally, studies show that obesity may exacerbate over pronation and heel injuries. Over pronation occurs when the foot rolls inward toward the medial (inside) part of the foot unevenly distributing the weight across the metatarsus. Weight is supposed to be supported by the lateral (outside) part of the foot directly below the ankle structure where shock is best absorbed. This cause undue pressure on the ankles and arches, weakening the structures and making the foot more prone to injury. Additionally, overpronation affects gait and abnormal twisting of the body to compensate for the uncomfortable weight distribution. This subtle twisting can lead to knee problems as well as hip and lower back pain.
Notice, I didn’t say that you should never walk barefoot on the beach, but doing so, I noted, makes you prone to injury. If you are willing to risk superficial injuries and you have had your feet evaluated to assure proper gait while walking barefoot, and you haven’t already been instructed by a doctor not to walk barefoot (i.e., you are already suffering from injury), then by all means, go barefoot. But to train on a regular basis without proper support and protection, is probably not something most podiatrists would look to kindly on. YMMV. 
I should have noted here that the rolling inward (pronation) is natural and normal. It’s when your foot rolls too far in that it causes an unnatural distribution of weight. I think I also mentioned the uneven, shifting nature of sand which can create a platform that causes you to shift your weight ever so slightly, as you roll through your step and the sand shifts, in order to maintain balance on a ground that is not steady and somewhat firm. It’s already very different than walking on lawn. It’s unbalancing in the same way walking on marbles would be, only to a lesser extent.
Depends on what you call “walking.” There is a sport, called “marcha atlética” in Spanish (my weak google-fu can’t find the English name) in which people walk at speeds upwards of 10km/h - that’s not the same as a walk in the park. It’s not jogging, the impact on joints is much lower than for jogging or running, but it sure isn’t “an amble in the park.”
If my back-of-the-envelope calculations are right, the current “top mark” in the Wikipedia entry (in Spanish, sorry) is for a guy walking about 10mph.
I can’t run to save my life (I don’t breathe right), but I can outwalk guys a whole foot taller than me.
Thanks for the explanation BEG. So he should be okay if he doesn’t over-pronate? And are you talking about “injuries” like stepping on a rock or a piece of glass?
I don’t want to come across as some barefoot messiah. By all means, find some comfortable athletic shoes and wear them. I’d be surprised, however, if it made all that much difference whether Sapo chose the most comfortable pair of sport sandals, court shoes, walking shoes, whatever in his price range. I guess at times I may go overboard in my resistance to what I perceive as people trying to make things more complex than they need to be.
One thing that may not have been mentioned - my understanding is that you derive benefit from mixing up your workouts to some extent. Doesn’t have to be drastic, just that if you do the same thing every workout your body gets used to it and derives less benefit. So maybe it could be a little as doing some lunges before, during, or after walking, jogging a short distance, or maybe walking backwards or hopping for brief stretches, climbing some stairs or sand dunes, lifting your knees high, or stretching out your stride. If walking on the beach, you can involve a whole different group of muscles by walking knee or waist deep in the water. But just don’t get in the rut of heading covering the exact same route day in and day out. Mixing things up makes working out more interesting as well.
Or supinate (the opposite of pronate; the outward rolling of the foot). Here’s some more info and visuals. Overpronation is more common, although both can cause problems.
Yes and then some. Those superficial injuries (cuts and bruises) that don’t always cause long-term damage to the foot structure and biomechanics. But superficial damage can either discourage you partaking in the healthy activities you like to do or make you compensate for them, thereby affecting gait and stride, which again can create pain and injury to other parts of your body. That would be a domino effect: superficial injury makes it painful to walk normally, so you overpronate which causes pain in the ankle, heel, knee, hip or back making your exercise counter-productive and not any fun. Or you can step on a rusty nail necessitating a tetanus shot or stub and break your toe keeping you off your feet as much as possible. Not a doctor, but it wouldn’t surprise me to hear that most superficial foot injuries occur while barefoot.
My point is shoes are generally better than no shoes for repeated athletic activity, and shoes that are designed for the activity you’re doing is better than just any random shoe. Barefoot has it’s place to, but there’s a reason why you see runners wearing running shoes as opposed to Chuck Taylors. Walking shoes are different than running shoes, as are hikers, cross-trainers, court shoes, water mocs, etc.
I’d agree with you and I mentioned scrambling (easy to moderate scampering over rocks) as an activity that promotes good balance and uses different muscles and some of the same muscles differently as a great low-impact companion exercise to walking. Many beaches have piles of rocks that are great for scrambling. I used to love to do that in Hawaii. Never been to PR, but I’ll bet the windward beaches are a bit rockier. You do need a shoe that maintains a good grip for scrambling, though, particularly on wet rocks. Having broken a toe, I can attest that it also doesn’t hurt to have a sandal with a toe guard or closed-toed sturdy shoe. Whatever the activity stretching before and after is pretty standard advice to keep the muscles healthy and reduce injury.
Does that help?
Calf stretches are pretty much essential to good foot health, and work better than most anything else for the treatment of plantar fasciitis. I speak from painful experience on this. Keeping your weight down will help, but is not a guarantee of freedom from this ailment. I know a lot of normal-sized folks who have bouts with it. Calf stretches are not strenuous and can be accomplished in the shower while you’re waiting for your conditioner to work. Or use a rolled up towel under the ball of your foot to gently stretch the calf muscle.
Another bit of good advice to prevent this affliction is to buy yourself a comfortable pair of house shoes. I wear topsiders (slip-on deck shoes) inside the house, as going barefoot really aggravates the plantar fascia. It also prevents you from breaking a toe when you accidently kick the chair leg.
I also highly recommend Keen shoes, and can’t say enough about how they really have saved me from a very sedentary lifestyle and possibly an early demise (I suffered from plantar’s for years). I wear nothing else on my feet (outside the house), even when exercising. Shoes are an individual choice, however, and you need to find the right one for you. These have worked miracles for me, but may be meh for you.
I have a #1 buzz cut, but I know what you are saying. I actually do them even while sitting in front of the TV now.