I think a lot of these altcoins offer improvements, but nothing on the order necessary to rival Bitcoin.  Litecoin, for example, offers a target blocktime of 2.5 minutes and the use of Scrypt which obviates the disruptive influence of GPU, FPGA, and ASIC mining.  These are both seemingly nice things in a cryptocurrency, and whatever supersedes Bitcoin may choose implement them, but there will need to be something radically new and innovative that compels consumers and merchants to actually convert their holdings, change wallets, etc.  Until then, why diffuse our efforts with all these carbon-copy altcoins when we have Bitcoin?  It's not perfect but it's the only one growing with enough momentum to have any kind of meaningful adoption anytime soon.  Alts like Freicoin, Namecoin, and Occcu are the exception, and as projects which are taking big strides in new directions I think there's every reason to be interested.