So long as it is above ambient temperatures, it i useful for something. Feeding low-grade heat into a stratified water thermal storage system is particularly effective. So long as the water is allowed to stratify by temperature, cold water will settle to the bottom and can be heated by any warmer material.
In practice, outflow for (re)heating is at the bottom of the tank, outflow for use at the top, and inflow is through a perforated vertical pipe allowing introduction at level of thermal equilibrium.
I don't disagree with you, I was just chuckling at the suggestion that heat that created the electricity could be "re-used".
As you note, waste heat is the heat that is left over after you've used some of it. The part you used is no longer heat (it may be electricity it may be kinetic or potential energy, it might even be chemical energy, but it isn't "heat" any more).
One of the coolest uses of ambient heat I've seen so far has been the new heat pump water heaters, they cool your garage as they pull latent heat into the water inside the tank. The trick though is that your garage gets cooler :-) (i.e. the heat has gone elsewhere).
There are also some pretty cool thermo-electric solutions where you stick a device on the indoors side of a window to the out of doors. It uses the delta between the window glass temperature and the indoor temperature to power a simple sensor. Not as useful on double glazed windows but awesome for single pane ones.
So long as it is above ambient temperatures, it i useful for something. Feeding low-grade heat into a stratified water thermal storage system is particularly effective. So long as the water is allowed to stratify by temperature, cold water will settle to the bottom and can be heated by any warmer material.
In practice, outflow for (re)heating is at the bottom of the tank, outflow for use at the top, and inflow is through a perforated vertical pipe allowing introduction at level of thermal equilibrium.