The problem is, if you support the correct standard, people who connect with the incorrect client will pester you with "bug reports", since things don't look right to them.
One of my early attempts at doing contract work was to write the database logic in some PHP pages, which would later have pretty buttons and such added by the web designer. Well, it turned out the web designer couldn't actually READ PHP code, and thus had no clue how to integrate their artwork into my code. Sooo, they tried to get me to do it.
Long story short, I developed what I thought was a pretty solid page, simple and decent looking, following their directions and mockup page pretty close. As a n00b to web programming, I was using Firefox. It worked great. However, the page didn't line up properly in IE, and as you might expect, arguing that IE was wrong didn't go very far since "everyone" uses IE.
If MXP support stays, there will be people using the bastardized ZMud client will try to add features they know it supports from other games, and submit bug reports when they don't work. If it's possible to turn it into a snippet, that would probably be the best choice. That way individual admins can paste it back in, and if they want to extend it for the psuedo-standard, they know where to poke.