I don't know the reason for the current limit on variable size instead of something larger, but it's not unusual to have some kind of limit. For example, until the newest version of Excel, there was a 255 character limit for cells if you wanted to use the cells in a calculation or other type of formula. Now, that limit has been greatly increased, but there is still a limit. The limit may have to do with the amount of memory variables use, but I'm not sure about that.
In any case, ~4000 characters is plenty for almost everything you'd need in mIRC. The only real exception is when dealing with very poorly formatted websites. And that's where binary variables are useful. Binary variables work very well and can be used when the ~4000 limit isn't enough. Or, just use a better website. I know that I will generally avoid a website that is formatted really badly (in code or on the visible site itself). If they can't code it well, then I don't trust the data or information to be much better than the coding.
Perhaps that limit can be raised again, but I really am not sure that it's necessary. You can do anything you need to do with binary variables, so there isn't a real lack of functionality; You just need to use different commands.
