Sure, but they should be upfront with that when booking. Fwiw the plane seemed fairly full when picking seats, especially when taking into account the blocked out the middle seats.
I don't think those seat pickers cannot be trusted for this information. I had one American Airlines flight that showed so full that I had to pay for seats so that my wife and I could sit together. When we got on the plane, it was clearly under capacity and there were plenty of seats together. I complained to the flight attendant a bit just to vent, and later I saw a refund for the up charge. I assume it was due to my complaint, but who knows maybe they have some sort of process that automatically refunds them on under capacity trips.
I used to catch a Delta flight back and forth from sjc to sea, that was consistently about half full.
I would always wait until everyone boarded, then move up to comfort plus or whatever it was called. On my last flight the attendant told me to move back since it wasn't fair for the suckers, no offense, that paid for it.
I offered to pay a nominal amount for the seat. The attendant pulled up the little register stick and said it would be $75. I said no thanks and moved back to my empty row about 3 rows back.
