I've been asked to swap 3 or 4 times and I've pretty much always said yes. Once the FA gave me something for doing it; I think it was a $25 voucher. I'm pretty sure I've asked to trade too. Yes, if I was asked to swap to a significantly worse seat I'd probably say no, especially if it was a long flight. I'm pretty sure that one of the times I was asked to swap for for a couple going to Bermuda for their honeymoon (I was going to Bermuda on business). I doubt there's that much difference of one MCE seat from another. Anyway, unless I think I'm asking the person to trade their much better seat for my much worse one, I wouldn't feel I was being a DYKWIA just for asking.

The thread did make me think though that there should be an FT button, lapel pin, etc. that would let us recognize one another.