DIALOG events aren't the same. It's reasonable to allow $input to work in a dialog event. The lashback on using $input in events has to do with events that are out of the users control, like ON TEXT, since it's impossible for a user to know when that might trigger. ON DIALOG can only be triggered by the user, so it's okay.
That said, you probably shouldn't use the u switch with $input in a dialog, because it's likely that "u" is only made to be associated with parent MDI windows, not parent dialog windows.