I agree with your point and have never been on an all-inclusive, but I do see one benefit. It pisses me off to no end getting completely gouged at nice hotels / resorts on basic drinks or pool food.
I was at the Ritz at Beaver Creek for skiing last year. For lunch outside they were selling burgers, chili, etc... everything was like $25. I get it, I'm at the Ritz and things are going to be expensive. But the chili was luke warm, the burgers were from frozen patties, and it took forever to get a beer. I'd be a lot more tolerable for those things if they were "free".