I'm not saying btcguild would ever do that, I just want to point out that it is not necessary to give them the opportunity by adding my miners to their pool.
And: As everywhere else in real life you should not feed the rich....
Can you please summarise what can BTCguild do if they hit 51% hashing power?
They can seize control of the network.
Watch this: https://www.youtube.com/watch?v=Lx9zgZCMqXE