I think people are missing a key aspect of the original poster's concern. What is said to children, over and over and over again affects them. It helps shape their conception of self and who they believe they can become. Particularly for girls it is a disturbing message to hear again and again that people are defining you by your looks. There are many troublesome components of this. As one poster mention often this attention is not distributed evenly in the family and and a child may hear every day the suggestion that her sister is prettier. Such frequent comments can feel like a kind of monitoring feeding into dieting among other troublesome behaviors.
A child will learn more from her parent's reaction to random people's random comments than she EVER will from those comments. If you want your child to be kind to strangers, you have to model that and not get irritated when random people are just trying to be nice.
I see it as a lot more complicated than that. In reality children are influenced by people other than parents. If we didn't believe that we'd just leave them in the care of random strangers without giving it a thought. For most teen girls I worry a lot less than they'd end up impolite, and a lot more about depending on these comments or feeling pressure based on receiving them. I don't think anyone is suggesting that the poster go about trying to model being rude, but rather there is a point where these comments can be damaging and it is appropriate for a parent to set limits.
Also, I think this may be a case where you have to have experienced it to really understand the matter of degree. The occasional comment is really different from being followed around in a public place or in the privacy of a family gathering.