The idea behind the whole "leave it on all the time" theory is this: the processor consumes ALMOST no power when it is IDLE. When your tablet is just on, with the screen off, in your pocket, it should be at an idle state, consuming almost no power. Here's the caveat: if ANY processor is being used (even just a measly 1% by some rogue process), then the processor will use almost as much power as it would if 100% of the processor was being used.

Overall, the battery consumption of the tablet being OFF, and being IDLE should be ABOUT the same. Since booting up and shutting down takes time, processing power, and therefore battery, why even bother?