I did switch once to accomodate a mother and daughter sitting together. It was equal seats (on a regional jet with no middles, both seats near the front within one row of each other).

I would be much less willing to give up a front aisle or window for a back middle. I might be willing to go from a back aisle or window to a front middle if I had a tight connection to make. If I had a middle, I'd be more willing to move to an aisle or window.

Hint to seat traders trying to sit together: offer your "better" seats (non-middles, front of plane) to the people sitting next to your "worse" seats (middles, back of plane) and ask nicely and you are more likely to get people to accept. The griping here, other than the "never change my seat" crowd, is probably mainly from seat traders trying to keep their "better" seats and stick other people in the "worse" seats.