Thats only for hobby miners who don't care about their electricity bill or don't pay it. If you're working seriously in mining, you have to factor in power costs. It's not 'almost nothing'.
Someday that will be a factor, but not for a while.
Let's look at the power hungry Avalon.
80 GH/s, 65M difficulty = $74/day income
700 watts, $.15/kwh = $2.5/day expense
Electricity costs 3% of the income. That's pretty low.
Or how about the Bitfury that is now shipping.
360 GH/s, 65M difficulty = $330/day income
250 watts, $.15/kwh = $1/day expense
Bitfury has 10 times the efficiency of the Avalon. .3% of the income goes to electricity. That really is 'almost nothing'.