You seem to have forgotten that Dorsett played a pretty large role for a depth player in both the divisional round and the AFCCG, with a touchdown in each, on the way to winning the Superbowl. Again, for a non-starting player, that's not an unimpressive playoff run.
In fact I could be remembering this wrong but didn't Dorsett have literally the only receiving TDs in the entire Patriots playoff run that year? I know we used our RBs, especially Burkhead and Michel, to finish off a lot of drives in both games and I know both teams worked very hard to cancel out Gronk and Edelman. We used our receivers to get field position but didn't get many receiving TDs in the playoffs that year, those 2 might have been the only ones we had.
They traded was at the time our 3rd QB to a team that actually had room to play him, and got a guy who turned out to be significant in a Superbowl run the next year. you can call it what you want but to me, that was a win win trade.