"If heating is your goal, thermal inefficiency isn't really a problem." - I guess my objection is based on an analogy with human-made devices, which degrade the more you use them. Thermodynamically it's fine to power up a computer to produce heat, but the inefficiency I really meant is that every moment the computer is running is another moment when it can go wrong. Perhaps the brain isn't like that: perhaps the brain's activity doesn't change the mind's lifespan. Or, more prosaically, perhaps the mind does degrade faster if it's active more, but evolution didn't care as long as the meatbags kept pumping out their genes.