There are degrees of mindless! I know one can't substitute watching lectures for serious study, but it's a great substitute for the far more mindless and addictive content I would be tempted by otherwise. There are lots of interesting topics in the foundations of physics and areas of mathematics my undergrad didn't cover that came to my attention this way. I enjoy getting a sampler now while I don't have the time/energy after work to study them for real, and they will inform what I study when I get and/or make time for them in my life soon.