I'm not bothered by this minor UI issue you describe, though.
Well, allow me to argue that this minor UI issue is in fact what deserves the focus here. The point of fixing this minor issue would be to make clear to users that the "User ID" field is not the place where they should set their preferred ident value if they have the "use ID from email address" option checked. This may make them actually look at the other fields on that options page, and see why having that option checked AND leaving the email address field blank is creating a problem for them. They can then either uncheck the option or fill in an email address.
After all, there is no actual bug here - mIRC just makes one particular choice for its fallback option. Rather, this is about about certain users having wrong expectations about mIRC's behavior; in other words, a usability issue. I'm not saying this to downplay the problem you're presenting here, because usability is incredibly important, and in my opinion not something mIRC always gets right. Thus, I think it makes more sense to solve the issue by improving the usability aspect underlying the issue in the scenario you're describing. In contrast, choosing a different kind of fallback behavior would also resolve the problem in that scenario, but not the underlying usability issue, and thereby creating or not solving problems in other user experience scenarios (there are probably more appropriate terms for this, I'm by no means an expert in the UX field).
Note that I'm just presenting an alternative view on the same issue here, nothing more.
