Many years ago when I was flying 100,000+ miles per year (first on American, then Delta) I learned what you got from the frequent flyer miles. As you note, the dirty little secret is that once you've got all those miles, the only place you want to go is your couch.
What the programs did give me were better status with the airline and that translated into better service. Lots of upgrades to first class (very much appreciated -- I could actually get work done in first class). Once when I wanted to take the missus with me to a conference, using frequent flyer miles for her ticket, the AA reservation clerk told me "There's no available seats for frequent flyer redemption on that flight. You're a platinum member -- hold on a minute while I free one up." There weren't any seats available near the front, so "hold on while I release a pair of seats." Thank you very much...
When I left my daytimer on a plane, they delivered it to my home the next day.
The chocolate chip cookies that AA sent me for Xmas one year weren't a big deal, but they were very nice...
When travel plans go wrong or schedules get messed, having preferred status on the airline means you might get the seat instead of someone else.
That said, I'm very thankful that I seldom travel these days. 5 days a week on the road, 50 weeks a year, is just no fun at all.