It happens all the time with closers moving to other teams and getting put into set-up roles. Look at the Brewers last year with Rodriguez. They already had Axford who was doing pretty well, so there was no reason on paper to get another closer. But the Brewers probably thought it was in the team's best interest to get a guy who not only could set up just fine, but be a nice back up in case something happens to their main guy.
I could see a team that trades for Myers thinking the same thing, or even using him as an emergency starter/long relief. Depth is always a good thing when you're in the playoffs.