Daily scooter rider here (through NYC rush hour traffic almost every workday), though I ride a Vespa it is a GT200 with 12" wheels, not the 150cc ones with 10" wheels you specifically mentioned. Still, it is clearly smaller and less powerful (at 200cc) than most “standard” motorcycles, and with smaller wheels.
I did take the MSF course, where I was allowed to ride my scoot but elected to take the chance to learn to ride a standard motorcycle. It was fun and I thought about getting one, but stuck with the scooter due to the nature of my riding (almost all in the city, and almost never on the highway). Yes, I can countersteer and lean in turns and all that jazz on the scoot.
I always ride with a full face helmet, motorcycle gloves and jacket. I do see many scooterists (especially during the summer) riding around with half-helmets in shorts and t-shirts and sandals like it’s a big skateboard or something. These people are in for a world of pain if they ever take a fall.
…as I did just two months ago. An SUV cut me off and I hit the brakes, managing to avoid rear-ending the car in front of me, but unfortunately also instinctively turned the front wheel to the side as well, which immediately threw me off the bike at about 35 mph. (The MSF teaches that you should keep the forks straight while making an emergency full-stop brake, but the instinct to flinch away from impact – and to also steer in the direction you are looking – is very hard to overcome.)
I hit the pavement with my hands and knees, slid 20 feet or so on my chest, and took a bit of a bump on the side of the head (I dove for the curb to get out of the street in case the car behind me was about to run me over – fortunately, the driver was paying attention).
All the stuff I wore did its job in keeping me only in bruises, except for my right knee which got very ripped up (my khakis did not hold up against the asphalt, needless to say). The leather palm of my right glove was shredded, and the carbon fiber knuckle guards ground down by a couple of centimeters – what if I’d been barehanded? My Kevlar exterior jacket had a scratch pattern about 5 inches long down the front – what if that’d been just a cotton T-shirt?
(I now also wear knee/shin armor under my pants.)
Would this have played out any differently had I been on a “full” motorcycle? I don’t see how. Braking distance is probably longer on a larger bike (such as a Harley) due to the greater weight. The GT200 is 315 lbs. and has dual single disc brakes; a Harley Sportster 883 (the smallest in the fleet) is 550 lbs., also with dual single disc brakes. I haven’t looked up the respective braking distances but based on those two figures alone, I would guess the scooter has a shorter braking distance.
Being bigger and heavier lends stability when riding at highway speeds, and the idea of being able to “accelerate out of trouble” is also a highway rider’s thinking; my scooter tops out at about 70mph, with very little acceleration available beyond 60mph, and the occasional highway stints I have had on it have been pretty nerve-wracking. On the other hand, if you have a full-out accident on the highway at speed, you are in big, big trouble no matter what armor/helmet gear you have on, and riding a bigger bike will not help you.
On the other hand, most motorcycle accidents happen in city driving, the most common ones involving cars making a left turn right in front of you because “they didn’t see you”. This is equally as likely to happen to you on a motorcycle as a scooter, and here is where the gear really does its job. That Steelers QB who shattered his face in exactly this kind of accident recently would have done no better or worse on a scooter, since he was riding at what would be a normal speed for even a 150cc scooter (45mph or so). What mattered was that he was not wearing a full-face helmet (those silly half-shell helmets don’t protect the face at all).
In city riding, my 200cc scooter has plenty of acceleration to get away from almost any car, and the smaller wheels also make it far more maneuverable at low speeds in weaving through stopped cars. For me, gridlock is something that Happens To Other People, including the guys on the Harleys.
Finally, most modern scooters have CVT transmissions, providing clutchless “twist-and-go” riding. No worries about stalling out in an intersection at low speeds, or taking mental energy away from constantly scanning for taxis, pedestrians and bike messengers to worry about being in the right gear.
If I ever had to do significant highway riding I’d probably get a Suzuki SV650 or a Kawasaki Ninja 500R. But since I basically ride only in the city, I find the scooter is the perfect two-wheeled motor vehicle!