It's irresponsible for mIRC to go around adding random control codes for arbitrary text style modifiers. Everytime a control code is added, other clients need to add it for compatibility purposes. In addition, mIRC needs to make sure that other clients don't already recognize the control code for something else. This is/was actually a real problem for the newly implemented italic code, which isn't displayed correctly under certain IRC clients. I'd rather not see problems like this repeated, because it makes things much more complicated for IRC client implementors.
Furthermore, there are only so many control codes available to be used. I would imagine we would want to use them sparingly, and not waste the non-printable character space codes that aren't really used. This matters, because once a control code is implemented in multiple clients, it can't be taken back. We will have to live with it for the rest of IRC, just like bold,colour and underline. Note that this is a real problem too-- mIRC currently cannot display these characters in the editbox in Vista and 7, and there is basically no solution for the near future because of the choice of codepoints for the existing control codes. This problem, of course, could not have been foreseen, but let's not make even more of the IRC experience depend on this non-portable, problematic, syntax.
IMO with all these complications, I don't see it as worthwhile to add in something typographically esoteric/obsolete like strikethrough. Even HTML5 is deprecating <strike>.
