It's also not a gaming system, yet people make games for it. They also make 'notepads', and mp3 players, when its an IRC client. Should /splay be removed from mIRC too?
Exactly, they make
those things. Just like they can make a spell checker... I mean, those things you named aren't built into mIRC, so why should a spell checker be built in? It can easily be scripted, or someone could just download a script...
If all those things were going to be built into mIRC, why not just make mIRC a full fledged operating system?