Players get traded all the time for various reasons. Some coaches and GMs can see value in a player even if they didn't perform well for their last team. We just traded Lutz and Trautman last year for late round picks. It's not uncommon to trade players that under perform to teams that have less talent.
Now if you're talking about under performing players with huge contracts, that points back to the question of why we gave them those contracts in the first place. Which is also a GM cap management issue.