Most of the singers on Idol are reasonably good, but only Chikezie could match Leona as far as vocal ability(on a technical level and for different reasons), although he is not as polished as she is and has less control of his voice.
Also, she won a reality show.....they are competing on one.....she is one of the most successful winners of that sort of show.....that seems reason enough to have her on, and if she was not promoting her album, someone else would be promoting theirs.....who can blame her management for wanting to reach a huge portion of her potential fan base ?
Mariah has more than proven herself, has an amazing voice,and any one of the contestants would give anything for a fraction of her success, so she is justified as a mentor/performer as well.
as far as 'most of the singers can sing'...um, most drivers can drive, whatsyerpoint?