I think we want bitcoin to be stable before all else, so if somebody wants to use different chain or different network, perhaps simply a forked client should be made for that.
The test network (run bitcoin with the -testnet flag) is a different block chain, implemented so that the two chains never get confused. If anybody wants to start yet another block chain, it isn't technically hard, and won't cause any problems for "real bitcoin".
The hard part would be getting anybody to use your alternative; it would have to be better in some way other than just "I started the new chain, so I've got a lot of the easy-to-generate-at-the-beginning-coins."
Well it's good as long as the user is required to enter some "advanced hardcore settings", but it shoould never be in a way, where You install bitcoin, and the first thing that pops up is "would you like to use the official chain or one of the less-official chains X, Y or Z" window.
Sure, official client can support multiple chains, but only through some hardcore/advanced configuration option i think. Also, multiple warnings like "do you really know what are you doing" should appear if user chooses another chain.