That is the problem... once you move your DH into the field, the pitcher is forced to hit.

If you plan to pinch hit for the pitcher later, then you'll have to pull him from the game, pinch hit for him, then replace that hitter with another pitcher in your lineup.

Of course, you could always move the pinch hitter into a position.... but then the place where the replaced fielder was hitting would be occupied by the current pitcher.

Trust me... it isn't that hard to follow.

Essentially, once the DH moves into the field... it can never be reclaimed.