I think you are missing a bit the point.
If we don't know, we still have a good idea about how things work, either because we tried to do it, or simply because it's part of our knowledge.
If Khaled were to add it, it would most likely bring up bug reports, meaning Khaled would have to spend his time on them (which is, as argv0 said, not needed, just a nice feature), rather than spending his time on mIRC itself.
A lot of people, probably Khaled included, prefer to see SSL working than syntax highlighting, this is understandable.
Also, the coloring part of the richedit control in the script editor is a slow task, that feature really represents a big amount of work.
Syntax highlighting is probably somewhere on Khaled's todo [huge] list, but its priority is probably not very high.
It makes code easier to interpret. Mistakes are easier to spot and correct. And it helps users who are new to the language better learn it.
That's exactly why you have mslDev, it already does this better than anything you will ever see
