Remember when you were little and you would see one of your teachers at the grocery store and it seemed so weird to see them out of "their setting"? That's kind of how I feel about all these singers showing up to judge reality shows.
I know they're not being "themselves" when they're on t.v., but still I would rather listen to Mariah Carey, Jennifer Nettles, Christina Aquilera et al than see them all tarted up and being forced to groom their replacements.
It's like when some tech person has to train the people who are going to take his job in India. Why would Mariah WANT to do that? Either she's really secure in her diva position, or she's looking at the big paycheck.
I'm sure she'll be fine. Save the show? Nah. I'd rather just turn up the radio and listen to her warble.