You don't actually need the users concent, I just connected to a server twice, one connection using the nickname 'scripts', messaged my other nickname and it opened the same desktop window when I clicked on the query.
Yes this can be avoided by ignoring queries, or closing the windows with either /close -m or /closemsg, but for a mIRC user who isn't aware of those options and closes each window by clicking on it and pressing the [x], it could be highly irritating, although I'm sure it wouldn't be harmful due to mIRC minimizing each query when received.

- Jason