On Calling Lets

This is from the USTA site question and answer section.  This was a good question regarding an opponent calling a let after already hitting the ball.

Situation:

I played a doubles match this past weekend, where, at the end of a long point, my opponent hit a ball that was heading into the net and then immediately called a let for an errant ball from another court that was nowhere near any player or the field of our play. It ended up about two feet from the sidelines at the net on our opponents side.

Question:  Is it valid to call a let AFTER you hit a bad shot, if there happens to be an errant ball on the court?

Answer:  If the player has already hit the ball and then the player or partner calls a let, that it is too late. The let call needs to come before the ball is played by the player or their partner to be considered a hindrance.

If the call is done immediately prior to playing the shot, it is always difficult to judge if it was really a hindrance. However any ball in a player’s vision, even on the other side of the net, may hinder players during a point.

I know this is a sore topic with many of you, I've played in matches were there seemed to be a lot of lets due to balls coming on the court, but I've also played in matches were everyone did there best not to call a let due to a ball on the court unless it was a hazard.  Do players call too many lets because of "stray" tennis balls from other courts?