I think it depends on the circumstances. Snapping your fingers at your misbehaving children when you are on the phone to get their attention and shoot them "the look" is not rude, IMHO. It's a quiet way of not interrupting your phone conversation and getting the desired response from your children.
Quietly snapping your fingers at a co-worker as a "signal" is OK, as long as both of you have agreed that it will be your private, quiet "signal". I would assume that an agreed-upon signal would not be considered rude.
Snappign your fingers at someone you aren't raising or have a pre-arranged agreement with, such as a waitress, is plain rude, and if I was a waitress, the last thing you would get is my attention if you snapped your fingers at me!