14 is High School. Typically 9th grade, which is typically High School. I'm 15, and have an iPhone, and I pay my bill. Sure, cell phone's aren't a necessity, but in a world like today's, when they're usually quite affordable, why not have one? It's a lot better to have one and not "need" it, than need it and not have it.
If you say you "shouldn't simply pass judgement," then it would be a good idea to not simply pass judgement, like you said you would not do.
OK- early high school. Cell phones are "usually quite affordable," but since we're talking about the iPhone I think it's safe to can that argument. Since the OP makes no secret that his phone costs $300 (clearly in an attempt to garner more sympathy), it's obvious that he thinks since he paid that much for his iPhone it entitles him special privileges.
Anyways, the situation as I see it is this: A young adult walks into an Apple store with an aesthetic problem with their iPhone that a) is [probably] user-inflicted, b) is minor, and c) doesn't affect the ability of the unit to function. The young adult asks for a replacement, effectively trading a used phone for a brand new one, and the Genius refuses on the grounds that it's not Apple's policy to replace used phones with new ones on a whim unless there's something Apple can be held accountable for.
Since this is not what the young adult wants to hear, he becomes upset (further biasing his opinion of *all* Apple Geniuses) and asks to speak to the manager. This implies that he or she thinks they know more about the problem than the Genius does. This sometimes works if there's a special situation surrounding the incident in question, but usually not for run-of-the-mill scratches on plastic. The manager shows that the back of the iPhone can indeed be scratched by pocket detritus, and the young adult still doesn't like what they hear so he posts on the Macrumors forums (re: Apple Geniuses):
Dmac77 said:
They are the most annoying, and retarded people on the face of the earth.
Ironic, because I promise you that's what the Genius said to his friends when he got home that night about some of his customers.
While I am sure that not all Geniuses are helpful or courteous towards all of their customers, I do sympathize with their position. Their job, basically, is to deal with special hardware and software cases brought to them by a customer base that more often than not isn't going to know a thing about their problem except that it's not what they expect. Many times this will be legitimate- something makes a strange sound, something doesn't work like it should, etc.- but other times it's just a waste of time. Like this incident, for example.
To the OP: I could offer a better informed opinion if you posted some pictures of these "cracks," but like I said before I have an extremely hard time imagining plastic randomly cracking on a unit like the iPhone. Unless you've been baking it in an oven while you charge it, plastic is a very reliable material as far as resistance to cracks. Characteristics of a structural flaw in the plastic will be a very jagged crack, as opposed to a clean, straight crack that is usually brought about by enormous pressure exerted at that point.