There can be times where "what?" chant can be used to show displeasure of the promo, which is totally understandable. Instead of booing it can be used as a good chant to get under the skin of the wrestler.
However... There are times, and this is not just a few, where the "what" chant ruins a promo. They are trying to go on with a storyline, trying to do a promo, and while you are listening to the wrestler he gets the "what" chant suddenly. The problem is, if 10-20 people starts chanting "what" the rest follows straight away. As I said, yes there are times where it makes the promo fun, but there are times where it ruins the promo as well.