Why shouldn't mIRC use some good features of other clients?

As for the logic: a channel window for a specific network can surely have history, mIRC puts lots of logs in there, why would storing the last input from editboxes be different? For custom @windows (with an editbox) the history can be remembered by name. Using a specific window name again seems like a decent clue that it's the same window as last time, so the history can be relevant.

For the actual proposition: some people might love that their /msg nickserv identify password line is remembered when mIRC is started the next day. Others will probably hate it, or don't know about it and have someone take advantage of it.

Maybe a way to add entries to that list with a script? Then just write a simple on JOIN or on OPEN trigger to reload the history yourself. For getting the info: either a special $identifer or just use on INPUT...