There is a certain appeal to the idea; the continuity and safety in numbers. Sometimes the modern "nuclear family" seems a little lonely and isolated.
My own extended family is relatively tight, twenty of us total counting the kids and most of us live within 30 minutes drive. Last week my sister and BIL drove me to the hospital for an emergency appendectomy, and stayed with me all day long, since they were the closest available family (and my son doesn't drive yet, etc.). That's not unusual for us... but I know of families where brothers haven't spoken on the phone for ten years, nobody lives in the same state and each "nucleus" faces everything alone... I could see the appeal.