I've seen this mentioned before so it's not my idea, but I definitely agree with it.
It makes zero sense from a global perspective to have multiple Bitcoinesqe currencies. Each will be weaker than their combination would be. A small government could blast us away right now, or a medium sized corp for that matter. If all honest people work on the same currency we will be much stronger than if we are split.
There is one rather limited circumstance where I would like to create a second "Bitcoin" like currency that does make sense: within a MMORPG.
I'd like to create a game that uses a bitcoin-like currency (actually multiple "currencies") to be used as a means for "resource creation" within the game architecture itself. I detailed my ideas on this thread here: http://bitcointalk.org/index.php?topic=353.0
If anything, that is the exception that proves the rule, and I would have to agree that creating multiple currencies complicates the whole process. The only other possible exception would be to establish different network rules, where there is a substantial community that doesn't agree with the policies being implemented that have at least so far been arbitrarily set. The "right to fork" is always present, and if the community isn't happy with the rules there is always the possibility to set up different rules for say generating coins or for transaction fees.
I would hope that for now any sort of disagreements with policies and network rules could be resolved through a consensus process and a strong defense of why each of the current rules exist at the moment. It would have to be a major schism of the community and a political act to create a separate coin network that would have a chance to compete on a substantive basis. At the moment, I don't see that being too much of a problem and the concept of Bitcoins is so new that I don't see those kind of factions forming within the Bitcoin community that would warrant such a split. There are philosophical differences, but not enough for a fork.