In the future, it may even be more efficient to hook them up to a large computer that is able to use them as a power source, too. We could offer them a virtual reality completely indistinguishable from reality.
That would make a great movie. Perhaps not a great trilogy, but a great movie for sure.
The problem is it's hard to get right. I saw a documentary where people broke out of the dream and it caused all sorts of issues, they were able break out other people using pills and teach them to fly and other unrealistic things.