It leaves the conversation very much open to being 'polluted' by devious users.
For example:
If a person called 'nick_1' types 'yes I agree. <nick_2> I hate you all. You suck.' it would be impossible for anyone to tell if that was a real message from nick_2 or a fake one inserted by nick_1. Obviously for people using nick colouring it would probably be a lot clearer, but those who weren't could be easily fooled by it.
