Just like RusselB said, you can already create scripts using Notepad/TextEditor. Just as an example, when I'm helping someone write an easy script, I won't bother with putting it in my mIRC at all. I'll just write it on the board, and then they can copy/paste it and have it work.
The only difference is, if you're making changes to your script (with mIRC open, using your text editor), you will have to manually /reload (in mIRC), for the changes to take effect.
Where as, the script editor itself is built-in, and makes the changes automatically after you press the OK button. This is why most people beg for an updated "Scripts Editor"