Update: though probably you are right, it would cause problems (not only blockchain size, but also copyright infringements, propaganda of racism and other extreme cases of freedom of speech)... Probably better to have messages separate, maybe to integrate with IRC (AFAIK bitcoind had some code to deal with IRC already).
Perhaps that is a good idea for other reasons as well, but content-related problems would be solved by only embedding a hash, not the whole message.