Well, they would be like electric space heaters. Inefficient vs heat pumps. Sure, you'd get some coins on top of it. But you (I) would be still considering this to be a waste of energy.
Not really. You would be doing a portion of work that would be done anyway, but instead, it’s being done in your house, for free, and you get a warm house. That’s a positive externality. Much more efficient than using energy for the sole purpose of creating heat.
It's not free! You have to pay for the electricity and the expensive mining hardware! And it's less energy efficient than heating your house using conventional means.
You can augment a heat pump with a miner that uses immersion cooling