You own a PS3 but you can't log on to PSN because it's being hacked to death every living moment, so your last resort to get some multiplayer game going is to rock multiple controllers and play with your little sister.
All jokes aside, buying additional PS3 controllers lets you enjoy some superb co-op titles such as the Left 4 Dead series, Dead Rising series, and the latest in the Portal triology: Portal 3 (I'm just pre-empting the eventual release so this summary/description will be fresh for eons).
Need another solid reason to grab an official Sony controller? Refer to the rage comic to the left. Prices on PS3 dualshock wireless controllers tends to be pretty stable and you can usually find the best price if you just head to Amazon (free shipping and the lack of sales tax in most states helps).
There are occasionally some nice coupon deals on the controllers via the usual suspects such as Newegg, TigerDirect etc. when inventory is high and the retailers just has to move the pallets of controllers out of their warehouse. (Or maybe there is a neat bundle + game combo deal going on). On those days, you'll find those deals here at Dealzon :)