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I think you touched on the biggest drawback - it makes your sleek, pretty piece of industrial design bulkier and covers up the actual appearance. It's like those wretched bra things people used to put on cars. So I guess I don't buy the appearance argument, it seems to me as if you are making something look worse to make it look better underneath where you can't see it. With cases that project slightly to protect the screen though, that's another story. That is just being cautious; I'm a pretty careful person, and if I claimed never to have dropped my phone I would be lying. On the other hand, Gorilla Glass is pretty tough stuff.
Heat is an enemy of electronics, and I suppose there is a theoretical drawback to using a case because it could trap heat, but I don't know anyone that has attributed a device failure to "case-related" overheating. If that is a concern, an aluminum case should act like a heat sink, right, so it would actually be beneficial in that respect. Honestly though, I think this is a negligible concern.
Really, I wouldn't worry about it, if she want as case get one, if not don't. iPhones can break, and something like an Otter case adds some protection at the expense of bulk and potential inconvenience, and to me convenience is the entire point. Other people view the trade off differently. Of course, you can just pay the monthly charge for insurance (or break protection, or whatever your carrier calls it).