istjohn is right. Using a heat pump instead of resistive heating (which is basically what a data centre is) is many times more efficient.
That doesn't mean we shouldn't use the heat a data centre provides. It just means that it is not a good idea to neglect the development of energy-saving technology because the heat produced can be used somewhere else.
istjohn is not right. Energy produced by datacenters is simply wasted. even if you will use heatpumps there is still only so much the can produce; using datacenters waste energy would lessen the the power neede for her pumps.
Your point is valid, but that's not really what we were talking about.
You said that using data centre heat is an alternative to reversible computing. This is not the case. Neglecting efficiency improvements and instead moving the data centre to a place where the heat can be used is worse than making it more efficient and using heat pumps with the saved energy at a much higher efficiency. It is even worse if the heat is dumped into the air, as istjohn said.
Also using the heat to heat homes would be even better. The two concepts aren't mutually exclusive.
How exactly is using what is essentially free energy worse than using heatpumps? You will build datacenters anyway; it _will_ happen, irrespective of using or not using heatpumps. Using 10Mw of energy from a DS would mean that you'd need 2.5Mw less of energy for heatpumping.
the issue is that there is an upper limit to how much heat can be removed from a system each cycle, so even if you have a way to disperse the removed heat in a useful way you still can't grow compute beyond a certain point. And because scaling is exponential even immersing the whole rack into liquid nitrogen would only buy a few years of computing growth post-Moore's law.