There is a huge difference between the rules and the reality of the game, but the result is a lack of consistency.
For example, hand checking and other contact against perimeter players is classic problem. Clearly they are not calling hand checking on a consistent basis, but if not, what exactly is the NBA looking at when grading officials?
How much contact does it take for a foul to get called when a guy is under the basket? I've seen Amare get mugged by three guys with no calls while contested layups get called with almost none? It doesn't take clearly bad calls to change the results of a game, just leaning one way or another on ref discretion calls.
BTW, if there was going to be extra refs, I would suggest they go to two mobile refs and two guys right under either basket. The under the basket refs could specialize in watching for three second violations and the charge ciricle so the mobile refs can focus on the above the rim contact.