My guess, is that in most cases, things even out. For instance, if a pitcher throws over a lot, that could disrupt the batter. On the other hand, the pitcher may not be able to find the strike zone after throwing over multiple times.
If runner steals, then the lefty batter has lost the advantage of having the runner held at first. On the other hand, a base hit will now be an rbi. Of course, with 0 outs, batter may have the bat taken out of his hand and be asked to bunt the speedy runner to third.
This year, I saw a very speedy leadoff batter for several games. Kid was a force. If he got on with no outs, you could almost go ahead and put a run on the board. If fact, what I remember most is how often he didn't have to steal. Multiple times I saw pitchers throw over, throw over and throw it away and he would then end up on 3rd base without stealing.