Why exactly are alternate block-chains worthless? That's like saying Mac and Linux are no improvement over Windows (false, firstly, they may not be an improvement but they have different featuresets), so everyone should just use Windows. Competition makes the bitcoin devs work faster (wallet encryption is in I0coin!), producing better blockchains for all.
A similar question might be why you use bitcoin when it's a subset of namecoin: namecoin does everything it does and more, so why do you still mine bitcoins?