Personally, I think it is not as easy to find someone willing to swap. I know of quite a few people who have nice welled 28/29 fenders and are looking for non-welled fenders. Not too many folks looking to trade on that one!!
The other issue is many wells in fenders were either installed incorrectly or they have the repro well that is the incorrect shape so again, many are not interested in trading unless it is an original welled fender, ...which seem to be very difficult to find. Factor that in that finding two fenders that are the same condition to get an equal trade is sometimes difficult too from what I have experienced.