I don't like the idea of seperating different cryptos to different forums.
The community on bitcointalk is really impressive and not so many would register to another forums on a coin they aren't even invested in. Once you got into one alternative to btc you would have to be on bitcointalk and your alternative forum. That is no problem, but then there is no need to talk on bitcointalk about your 2nd coin and it wouldn't be seen by others.
If your favourite alternative happens to be PPC you wouldn't hear anything interresting on LTC so no need to even visit ltctalk and we would loose communication between the alternatives.