I agree with bruceba and CO 1E.

If I'm in a good seat and the potential-seat is inferior, I usually explain exactly that and suggest that they ask the other person to move (likely in the 2nd half of the inferior pair).

However, attitude goes a long way and I have gladly switched to an equal seat to accomodate. But I can tell you that if someone is already sitting in the seat presuming that I'll switch, they've started off on the wrong foot and I'm not likely to agree.