I read on here a long time ago that it wasn't just supposed to be humans as batteries, but as CPUs/algorithms (some good further discussion too, about the Architect): https://news.ycombinator.com/item?id=12508832

I thought I'd seen an earlier discussion on a mailing list or something but can't find it now.

I remember coming up with an idea, which I thought was original, of us being in a simulation where all our life choices are just training an AI advertising engine that then targets "real" humans, and then later came across this revelation about The Matrix, and of course it'd already been thought of.