The idea when Dante DTs is that he is using the power of that DA to give him other powers, and that is what changes his DT appearance. It isn't changing his base DT, but just like modifying it so some of the powers of the DT can shine through.
I can understand what you mean but I think it would be boring if Dante had just one DT, especially if it was ugly like in DMC4. Then again, if they have a DT like in DMC2, it might not be so bad. Maybe they could make it so that after you beat the game, you can have a DT costume selection, where you can choose the appearance of the DT throughout the game. That would be nice.