I don't see why a chain split is to be avoided ; I would say, on the contrary: the more it splits, the more different versions can compete in the market, and the better the outcome will be. [...]
In fact, in a certain way, alt coins do that already, but I think that it would be better, instead of starting new chains all the time, to fork off from existing ones, to have initial distributions which mean something. After all, to get something going, it is much better to hand it out to a large existing audience than to try to win a new audience.
I disagree somewhat. A chain split would mean a severe usability reduction. Every user that uses a certain number of Bitcoin services (e.g. exchanges, merchant sites, remittance services) is deprived from using a part of them in the case of a chain split if he doesn't want to use both chains. And using both chains is also a severe usability hassle (even if there were "multichain" clients).
Usage is what actually gives value to cryptocurrencies - without usage, they would be simply a software with no real use cases, because "low-to-zero" valued blockchains are easily attackable and wouldn't even usable as a Factom-style "notary" ledger.
So my stance is that alt-chains are a cleaner way to experiment with different implementations. If you are worried about the distribution of the supply, then your alt-chain can simply use a snapshot from the Bitcoin balances at a certain block (like "BTX" and "CLAMS" have done).
That's also why I support the agreement and only would support UASF in the case it has a real chance to get a supermajority of miners, exchanges/economy and users.