I agree it is best used for low-IP pitchers - I would add, to the uses people have already mentioned, that if you have a mediocre pitcher you got just to fill in a blank on the roster, cover the minimum IP or because you didn't have enough cash left to get anyone good after drafting your more important players, sometimes you can convert him into a half-way decent one by having him only face L/RH batters.
But everyone here is right - it is a pretty limited option, to be used only when it can serve. Drafting someone especially to be an LH/RH specialist is pointless, especially because the players are not rated v. LH/RH opponents, and the SIM gives a modest boost to batters facing opposite hand pitchers at most.