free does not mean unlimited, so when buying the miners it is good to take energy efficiency into consideration.
Lets say you have X amount of free electricity per month (limited by breaker, wire, street transformer, or other factor).
now lets say that X amount is totally free.
but you have to decide between a miner that uses 10 Watts per GH/s vs 1 W per GH/s, well is you use the miner that use 1 W per GH/s, given the same total amount of free energy you will get 10 times the hashing rate.
So free does not mean unlimited, so making good use of that electricity is a good idea, since it is an asset that you have, and the better you managed that asset the better.
Think of it this way the bitcoin faucet gives you free bitcoins, but not because they are free you are going to waste them because they are limited.
This is the very concept that gives bitcoin value, a limit of 21 million coins.
Given that electricity is limited, you have to consider all factors to make a good decision not just hashes per dollar.
Also getting rid of the heat is a pain, since the most efficient method is the use of fans a lots of ventilation, since if you use air conditioning you will be sacrificing energy that could instead be used for mining.
I know it is one of those thing that the average person will never get, the fact that something does not have a price, does not mean it has no value, even bitcoin itself when it started had no price, but it did have value.
if electricity is limited, which in reality it is, then yes I agree with what you said.. otherwise, I see no problem with my point.
The way I think about it as an economic optimization problem. You have an input and output, as well as a belief of what the bitcoin value should be, all you have to do is solve the optimization problem and see what hash per dollar *given* your electricity constraint and bitcoin value belief yields the maximum.
The average person may not get this, but it is not hard to educate.
