While I agree this feature would be great (currently I made a script which parses XML files using multi-dimensionnal arrays made of a hashtable), you should keep in mind that mIRC was intended for chatting.

So, if you'd like to parse XML files, you should probably do it yourself, use some workarounds (I've already seen implementations based on Windows' XML parser with $com calls), or use a MORE EFFICIENT language (a compiled one like C for example).