The supergroup. What a horribly dated concept that is. It smacks of corporate rock and overinflated, believe-your-own-hype egos, as in, "Hey man, you're the coolest guy in your group, and he's the main man in his band, and without me, my band is nothing, so like, if the three of us get together, man, it'll be an incredible band."
Maybe. Such thinking sometimes leads to artistic wonderfulness, as in Cream or (arguably) the Traveling Wilburys, and sometimes not, as exemplified by the ill-fated Blind Faith, where the whole was definitely less than the sum of the individual parts.
The supergroup concept has never really caught on here in Japan, which in many ways is a good thing. Tsunku, the Svengali behind Morning Musume, has been spinning off members of his stable of female "talents" into various new groups, of course, but these hardly qualify as supergroups -- the individual members of Morning Musume are so many peas in a pod, replaceable at a moment's notice with any one of the myriad musume out there.