It seems that the Ensemble is the current fad when it comes to TV shows. Look at Heroes, Stargate, Torchwood - they're all shows with four or more major cast members. The 80's and 90's may have been devoted to shows with only one or two leads, but the Naughties is taking things to the next level... as it were.
Ensemble cast shows have always been better and lasted longer. Why do you think soaps are still around? Night time soaps, like Knot's Landing or Dallas, were immensely popular. Other shows, like Hill Street Blues or LA Law, were also very popular. Many sitcoms center on groups, too. Currents shows have just taken this to the nth degree. The CSI shows, Law & Order shows, Heroes, NCIS, The Unit, etc. are all popular because they keep your interest by switching the focus. That's not to say that there are not shows that are interesting that focus on one or two characters, though. It just seems that Hollywood has realized that they can retain an audience with larger casts.