I will pretty much always trade F for F, even if I'm getting a "worse" F seat. I like aisle bulkhead but it's not that big a deal.
I do remember swapping to a worse Y seat a few years ago. I was working in Bermuda, and a couple who were (at least they said) on their honeymoon wanted to sit together.
When I fly Y it's most often 23F or 23A on the LUS A321 which have lots of extra legroom and I would never agree to give it up.
I've only ever flown lie-flats on the A332 and B763's, and I don't think I'd care much. But I always book a window and these are 1-2-1 so nobody wanting to "sit together" would want my seat.