It's not true that it causes no harm, it does. They are not just tags on that that topic you want to tag, they become forums elsewhere that propagate through a2k in fundamental ways and in all the other contexts are unhelpful.
I've explained multiple times how it does
cause others harm because it's misuse of a fundamental feature that is not meant to be used that way (simple example is that some other users have your graffiti tags as their "my forums" and the tags are simply not useful forums) but the truth is you don't really care anyway.
Tags create forums, if the tag isn't useful as a forum then it's not an appropriate use of tags and these tags will be filtered.
This is something that is going to be fixed as a high priority, and it's pity that the immature graffiti folks necessitate the work to clean up their graffiti.