Isn’t in not up to the pitcher, but the manager? When a team decides to intentionally walk a batter, I don’t believe the pitcher has any deciding that goes on.
Yeah, it’s a managerial decision. Back to your original question, there is little point in swinging in order to add to to the pitcher’s pitch count. The throws are usually not thrown with the same effort as a regular pitch. Think of the tosses like pickoff throws to first base. Also, the opposing manager can always bring in another pitcher. The better question is why have the pitcher throw any pitches at all – if a team wants to intentionally put a runner on first, why not just send him to first without any pitches? Well, baseball rules and tradition say no.
Also you will occasionally even at the Major League level see a passed ball or wild pitch in these situations, in which case any runners on base will advance.