I have stayed off site twice and I have learned my lesson.

Many of the shuttles only go to TTC or Epcot, so you have to transfer to go elsewhere. Many only have scheduled stops at certain times, so if you want to go back for a nap, you may not be able to. Many have timeshare people sitting in their lobbies trying to talk you into going to a timeshare seminar for 1 1/2 hours to get a park ticket.... that 1 1/2 hours can stretch out to 1/2 your day, and then they won't take no for an answer.
If you do stay on Disney Property:
1) save $8 per day for parking if you drive i
2) busses, boats and monorails that run from before the parks open until after they close to insure that you will be able to come and go from your resort at your convenience.
3) Yes, you will find DVC reps in Disney lobbies, but they won't approach you, you will need to approach them, if you are interested.
4) You will have a full Disney Experience, as Disney Resorts carry the magic back with you at the end of the day with theming of their resort rooms, pools, exteriors, etc.
5)You will be able to participate in activities, such at Extra Magic Hour, for no extra charge, which is not available to off site guests.
6) You will have choices of transporation at many of the Disney resorts, including the monorail (CR, Poly, GF), Boat Launch (CR, Poly, GF, WL, BC, YC, BW and to and from DTD from PO-R, PO-FQ, OKW and Saratoga Springs)

, and of course bus service.
I learned my lesson... never again will we stay offsite!