I have almost always stayed off site... I have been going to Disney since the year it opened and love it with all my heart. That being said, I prefer to save a little money by renting a house in near by Kissimmee.
With a large group you can rent a rather large home or two that have all the accomadations of home. Washer dryer, pool, ect...
And you get really great deals on them.
Yes the convenience of staying in the park is lost, but it also enables you to see the other parks easier (Sea World, Universal) or go to downtown Orlando and Church street station.
Also time is a factor. If you are going for a long period of 8 or more days the house will save you money and be more practical, enabling more money to do the dinners and shows you may want to see.
If your stay is a week or less then you may want to stay in the park just to be closer to everything.
Remeber the house cost the same regardless of the number of people. If it is $2000 for the week split into 3 couples it is a big savings vs. staying in Disney.