> Just knowing whether it is a fastball or an off speed is enough

Except if it's Mariano Rivera, in which case it's only one pitch: a filthy mid-90s cutter. The batter knows it's coming and is largely powerless to do anything about it (i.e. more broken bats than home runs were the outcome of hitting against him).

For starting pitchers, however, yes, quite useful for batters to have an idea of the speed of the pitch -- hard enough to hit a major league fastball, much less a splitter, slider, changeup, curveball, etc. on any given pitch.