I don't think investing in altcoins in at all different from investing in bitcoin, because ALL altcoins exist 'within' the bitcoin economy. You can only buy many altcoins with bitcoin, so if bitcoin rises this will automatically increase the dollar value of the altcoins as well. So if your money is put into an altcoin that is worth 100 satoshis, it will remain 100 satoshis most likely when bitcoin goes up, but those satoshis will be worth more.

Secondly investing in altcoins does allow you to benefit from both Bitcoin's rise as well as that of the altcoin itself. This can go either way of course, but considering the altcoin market is still in it's infancy (I think) it will inevitably be more profitable than investing in just bitcoin.