Calling him a DH in a couple years is a little rough. His defensive numbers are down this year, but from all I can see that's just random noise. As long as he can play a good defensive SS, he has a place in the field. Reyes is the guy who should not be in the field within a year or two, or who should be shifted to an easier position.
But it's definitely a trade meant to win in the next year or two, no arguments there, but that's the way the roster is built. The AL is still very open, so not a bad time to strike.