http://bugs.musicbrainz.org/ticket/3765 asks for the colon to be allowed, and there are other characters that could be useful. As I mentioned in that ticket, I'd like it if pretty much any character were allowed. If something is common enough, it shouldn't matter. If it's not, then it's no different from using any other weirdly spelt tag. Nobody's forcing anyone to use punctuation either.

Some examples that are weird because of stripped punctuation:
http://musicbrainz.org/show/tag/?tag=r+b (presumably r&b)
http://musicbrainz.org/show/tag/?tag=rock+roll (presumably rock&roll)
http://musicbrainz.org/show/tag/?tag=90+s+r+b (presumably 90's r&b)