I believe all nick completer scripts use mIRC's tab completer, so they'd need to somehow manipulate that for it to do what's asked. I think this could be a useful feature, though I'd get confused if I first did "De TAB TAB" to get to the right person, then the next time did "De TAB"... I'd be likely to hit TAB again and then have to scroll through everyone else that matches just to get it right. Of course, I generally type enough letters to get the right person on the first TAB anyhow.
Yeah, I'm sure some would prefer the functionality like it is, which is why I would suggest that there be the option to keep it as-is, or to use "smart" complete.