I see this explanation a lot, and I get the logic, but as someone who almost always has a car (mostly rentals, we have driven to./from twice) despite staying onsite at
DVC, let me suggest that you
might not find yourself driving offsite as much as you think. No guarantee, of course. Everyone values their time and money and the friction of using either differently. But I'm not driving from BLT or the Poly to an offsite drugstore to save <$10 on a couple of bottles of sunblock. That's easily at least 45 minutes round trip, maybe more, not counting time in store. On our last trip I forgot to bring a Britta filter for the pitcher in our Owners Locker. Those are not sold onsite. I ordered a pack from
Amazon and paid the overnight premium on Prime rather than drive to an offsite store where I could buy them both immediately and for less money.
One big planned grocery/sundries trip can make sense, and driving for a better/cheaper restaurant option also. But little stuff, no. And the biggest obstacle to splitting Disney and Universal is not transportation, but that 2 extra days at WDW (once you buy the first 3-4) cost much, much less than the first 1-2 days at UO.
Now, if the lack of onsite benefits keep you offsite altogether, that's a completely different analysis, of course. But I think people who rely on Disney transportation overestimate how much they will venture off with a car. My family has more than 10 week long onsite w/ car stays in our history. We twice did one day at UO, to experience Harry Potter, and exactly once went offsite for dining (which involved seeing a family friend who lived in Celebration), and that's it. Only other trips were to attend Mass, which is (almost always) not available onsite and not vacation related.
Again, I acknowledge that everyone has different priorities, but once you're onsite at that massive, nearly all-encompassing resort, the mental effort to decide "let's go elsewhere" is pretty high.