I've traded a few times at the request of other pax, or of an FA, to unite another group or pair of pax. I've asked others to trade once or twice so that Mrs. Law Lord and I could sit together (as noted above, usually an issue only when we get upgraded).

I don't mind at all being asked if the person who asks doesn't mind if I say "no." About the only time I can think of that I've said "no" was when the traders were willing to consider a trade in one direction, but not the other, and appeared to be willing to trade only if the trader also got a better seat. But unless the traders were assigned two middle seats, whoever's sitting in the middle (or otherwise less-desirable) seat next to one of them is almost always willing to trade to a better seat.