Why is HTML in forums so dangerous?

[bairy]

In the forum manager // Allow HTML // Admin Help it says

This allows users to use HTML while posting. It is strongly recommended that you DO NOT turn this on as it can severely compromise security and/or severely mess up layout if users insert malformed HTML. Even if you set this to Yes, users still cannot use certain tags, including javascript: and about:.

Aside from screwing up the layout, why is it so dangerous? And couldn't the vb team put a filter of 'safe' tags as other sites (blogger, livejournal) do?
I know you can use vbcode and that's fine, and you can make new code to cover tags such as <strike>. It's just sometimes a <table> with css would be really useful in a post.