Unfortunately there are just too many reasons NOT to have this in mIRC - I highly recommend using the search feature at the top of the forum to find out just some of the points that have been mentioned before. Expand to 'Feature Suggestions' and 'All Posts'.

Some issues that have been raised in the past:
- Different languages - how should mIRC interpret them? Special characters?
- The time it would take to compile all of the world's dictionaries into mIRC.
- Abbreviations - distinguishing between 'lmao', 'rofl' and so on.
- It wouldn't increase people's ability to spell, it would just have a machine do it for them.
- Different types of English - UK/US have different spellings, some completely different words.
- Typos are an acceptable part of the IRC world and are just a part of what makes IRC....IRC smirk

At this time I find it unlikely mIRC will have a spell checker put in. There are addons out there which have a go at it - check some of then out here.