Well, it's not really investment. It's called speculation.
Not that there's anything wrong with that.

As for altcoins, 99% of all the interest around cryptocurrency is generated by a post-event hype of a successful get rich quick scheme that made millionaires of several early adopters (and it's quite clear that the Bitcoin train has left, you can't early adopt it anymore, the rocket fuel's gone).
Most altcoins come from the misguided hope of duplicating that success. Of course, the rate of success is unlikely to come even close to Bitcoin, but it will have a profound negative effect over time to the very idea of cryptocurrency as a whole. It's as if in the late years of the California gold rush, miners could just make up new minerals that are still abundant - and there would be people around to buy those at a premium.

However, given the inherent flaws of Bitcoin, I am sure that if cryptocurrency does go mainstream, it will not be Bitcoin, but some alt that has addressed those flaws. PeerCoin is an interesting step in the good direction, but its money supply scheme is still not inflationary enough to be useful as a currency, and in fact encourages hoarding to a ridiculous degree. Also, the transition from proof-of-work to proof-of-stake operation sounds shaky at best.