You don't always have a prospect for each position at each level(if you do, you're the best HBD owner ever). If I have a SS who'll never be more the minor league fodder and a DH who'll be a top BL hitter(but on a NL team), there's absolutely no reason, assuming you buy the development process that's being sold, to not play him at SS. You'll want your DH to be the best he can possibly be, because he's going to end up at 1B, and your concerns over your non-prospect SS are virtually nil.