There are a couple situations where you want copper; actually copper is somewhat my specialty - check out my effect with and effect without on it, it's sick. Actually it's really sick. Anyway, here's basically when you'd want to open copper:
When you're going to buy a lot of green cards very fast - this includes some (though actually not THAT many) gardens decks, and, quite importantly, most duchy/duke decks, where copper is a very good card. It can be good for some decks where you're opening potion and trying to make sure you get up to say 3p - say you get 5/2 and familiar's on board without any other particularly good alternative (like another curser). Finally, there are some copper-driven strategies. Possibly coppersmith, moreover if you want a counting house or apothecary deck you may well want the copper even from the beginning. But honestly, all these situations together come up so infrequently... I mean, it's maybe 1% of games, and then you have to get 5/2 start, AND not have a good 2 to buy.