First of all, in real life pro tennis matches before Hawkeye, it always infuriated me when the linesmen made a bad call on an important point, or the chair got involved and wrongly overuled from the opposite sideline...

My point is that Hawkeye is great for real-life tennis matches, but I'd much rather have my computer tennis games without mistakes (real or pre-programmed.)

Besides, as has already been pointed out, it would only slow down the online games (especially the way that so many already seem to abuse stuff/cheat and so forth) and the time and money could/should be more wisely and practically spent elsewhere in game improvement...
