That said, if this field really is only used if the identd check fails, then it shouldn't be there in the first place. mIRC as client should be able to just use any other already set uservalue (like the primary nickname) as identd replacement instead. That way users are not confronted with fields that don't do "what's on the label".
Wrong. Users must be able to control their ident because it's an important part of the hostmask-- and identd is not available to all users. It's the only user controlled non-unique part of the hostmask that can be used to verify identity (pseudo-reliably) even when the user has multiple clients connected or has their primary nickname in use by someone else. I definitely would *not* want mIRC choosing my ident for me. I actually set [x] Use ID from email address
in my identd settings so that I can supply per-connection identds for my multiple networks. This is one of the main advantages of the email field as an identd. The one identd field in the identd pane cannot accomplish this.
Maybe the field name of "email" should be updated to something more relevant (and not require an @host part), but it should surely not be removed. Frankly though, this is rarely brought up, and I don't think it's that big a deal once you get used to it.