No matter what the level, it is considered rude and disrespectful, otherwise it would be used more. Whenever you use it, you insult your opponent, that´s just the way it is.
It's true and the fact that it is legal is a glitch in the rules.
Just like in table tennis, it should be legit to hit a serve only if the ball has made a certain height in the air.
Just because the oponent can't be ready for such kind of shot and despite that this is the point of the surprising serve, it would be just as "legit" to serve fast while the oponent is still whipping his face with the towel facing back to the net.
That's the same as the under arm serve imo.
If one is able to make a drop shot serve with a ball thrown in the air as for a normal serve, then it's cool, but otherwise it's just kinda cheating.
I'd expect such serves from Benoit Paire in the future, he is the kind of wild tennis player with no respect for unwritten rules