I think, as with most issues of etiquette, it’s simply a matter of context. For instance, if I tell my waitress that I want to buy a drink for a woman on the other side of the restaurant, I don’t think it would be considered rude for the waitress to point me out when delivering the drink.
If, however, you are walking though the grocery store with your six-year-old son, and he points at someone with an obvious physical or mental disability, or some embarrassing trait—that, I think, would be considered rude. Especially if it is accompanied by the six-year-old voice (same decibel level as a jet engine) saying, “That woman has a huge butt!” Not that I’m speaking from experience. Actually, I am.
So omfgTALIjustIMDu is right—it’s rude because our parents didn’t want to be embarrassed by the social ineptness of children.