Just as a simple example of what looks like simple code; as an admin go ahead and post a HTML post just including the following:
Notice that by having a stray close table or div tag how your entire forum is messed up on any page displaying this post (the thread, portal news, active topics in this case); this can happen with HTML posts... This is just a minor display problem (and these are two common tags for HTML), not even touching on potential security issues.
I would be surprised to hear a legitimate excuse on why a normal user would need HTML posting access on your forum that can't be solved with UBBCode; all of the formatting coding they should need is represented by UBBCode, HTML/Web Design tools aren't generally something one would need to contribute to a conversation.