Ok, sufficiently baited to do an actual response 
In addition to fully agreeing to brinebold (e.g. stress on GPU), even the electricity cost is a relative thing.
Personal example:
I am living in a house which (typically french I guess) has electrical heating only. Most of the rooms are heated by one electrical radiator per room, the total electricity the heating eats up in the winter months is about 3kW total.
I could replace the radiators in a few rooms by mining rigs at 0 electricity cost. Any W they use ends up as heating in the end, saving me the same W in electrical heating.
So even if GPU mining drops to the point (very location -> electricity price dependent) where it technically doesnt look profitable power cost wise, 5-6 months of the year I could mine for no running costs at all.
Whether one thinks its worthwhile to buy rigs or upgrade machines specifically for mining is a different question, which everyone knows best himself i guess.
P.S.: I wrote a lot of "could", the above actually was my plan for last winter, as I have a few slightly outdated PCs standing around (that can still fit 5790s), but then was too busy with other stuff to actually set it up
In addition to fully agreeing to brinebold (e.g. stress on GPU), even the electricity cost is a relative thing.
Personal example:
I am living in a house which (typically french I guess) has electrical heating only. Most of the rooms are heated by one electrical radiator per room, the total electricity the heating eats up in the winter months is about 3kW total.
I could replace the radiators in a few rooms by mining rigs at 0 electricity cost. Any W they use ends up as heating in the end, saving me the same W in electrical heating.
So even if GPU mining drops to the point (very location -> electricity price dependent) where it technically doesnt look profitable power cost wise, 5-6 months of the year I could mine for no running costs at all.
Whether one thinks its worthwhile to buy rigs or upgrade machines specifically for mining is a different question, which everyone knows best himself i guess
.P.S.: I wrote a lot of "could", the above actually was my plan for last winter, as I have a few slightly outdated PCs standing around (that can still fit 5790s), but then was too busy with other stuff to actually set it up
I'm going to pick some nits here, since you're invoking the 1st law of thermodynamics in your argument. While energy in a closed system can be neither created nor destroyed, your house is not quite a closed system as the walls do not prevent the escape of all energy. We'll ignore the loss of heat because you'd be losing it at the same rate if generated by an electric radiator too so that's net 0 in this comparison. However, you're going to export some of that energy to the internet in the form of data packets. I haven't quite measured it myself but I'd imaginee proof of work takes up at least as much bandwidth outgoing as incoming work requests. You'll also be leaking energy through your walls in the form of EM waves (more if you use wifi) that pass through our walls without being absorbed and converted to heat.
Accounting for what probably amounts to less than 1% of the energy in use here, your logic is still sound. I have gone months later than most people before turning on a heater because my apartment is well-insulated and I have two computers running in my bedroom so I know processor-based heating really works.
If you had done some more general assumptions instead of the nitty-pickings, I think you would have had a better argument.
To use the rigs as heating and counting the electricity as 0, one has to put the rigs in a room where one would normally would use at least the same amount of electricity for heating.
This means that one get a lot of fan-noise in your living room etc.
My guess is that most people put their rig in a space that would normally not normally be fully heated in the winter. In a storage room, hallway etc. If so, there will be extra electricity needed to get the same heat-output in the living area.
Besides - A heatpump would have given the same amount of heat for less than 1/3 of the price.
Still, If it's cold outside, and you can reduce your heating bill due to rigs, the reduced need for other heatsources can be calculated against the rigs electricity usage.