Moderate FF here (usually a little over 100 segments a year).
Yes; most frequent flyers try to stick to one airline because that’s how you rise up the status rank for a given year, and only on that airline will you see the benefit of being a FF.
The FF miles programs are largely loyalty programs for a specific airlines. Some CCs give you airline ticket credits for any airline, but that’s really just a way of them giving yo something that looks more valuable than it is, and you will be restricted to flights/fares they are willing to provide… You will see far fewer benefits if you collect mileage from many different airlines. Most actual frequent flyers are more concerned with their status rank (it’s literally called that) than the free flights that cashing in miles gets you. However for a true FF, those miles can purchase expensive tickets. For example, I’ve cashed in 160,000 miles for a $29,000 international first class ticket.
In general (and not including Southwest, which doesn’t have a First Class), the major benefit for most true FFs is seating (and SW does have some advantageous seating; just not First Class). As you progress up the loyalty rank you typically get:
- More free upgrades from Coach to Business to First. (Most domestic flights are only two class.)
- Higher boarding status. Get on the plane before others. Good for making sure your bin space is still open. Useless for anything else other than feeling grand.
- Better selection of Coach seats; Exit rows or seats with more legroom.
- Free luggage
- Free alcohol and snacks, even in Coach
- Special phone number for flight assistance, including rebookings
- Priority rank for requests such as standby flights or seat changes
- Discounts on airport club memberships
- Priority seat selection and boarding for traveling companion
- Security lines and airport check-in specific for those with high status.
All of this makes my flying simple and convenient, but that wouldn’t be the case for anyone not flying often enough to get to the top status. I seldom take more than a minute or two to get through security at any airport with TSA Pre; seldom more than 10 minutes anywhere. I almost automatically get upgraded to First. But the flip side is that I have to be a bulk buyer of a single airline to get that. In my mind you either want to fly all the time and get high status on one airline, or fly almost never and avoid the irritations of the airport life.
Each FF program has its nuances. At AA, the highest level (Executive Platinum) gets you 8 upgrades even on overseas flights. That’s huge, comfort and dollar-wise.
If you don’t actually fly frequently (say, at least 30 trips/year), then a FF program is pretty much just a mileage program that accumulates year over year until you have enough for a free flight. I don’t think saving mileage for a domestic free flight is really worth much, but it’s not negligible.
If you want free flights, I think Southwest has the best deal. As a rule of thumb, focus on a single airline. For any CC offers, or other “points” programs, just calculate out a dollar value for what they are giving you and decide if it’s worth it for you. In general it’s not, but YMMV. Literally.