While the switch makes sense, Do you think that having such an option is actually going to change the existing poorly written scripts or unknowledgable users? IMHO they'd either a) ignore you, b) fumble and fail to fix the command or c) listen by stopping the use of amsg altogether. I personally don't think the switch will do anything to curb the misuse of /amsg by people who clearly want everyone to see what they're saying.
That said, for people who *do* know what they're doing, having to ignore one or two channels is quite common and relatively annoying to have to script.
