I hear this argument all the time. But unless you're doing something that's solar, battery, RTG or otherwise non-grid powered, then what does it really matter? At what scale is a person running so many old laptops that they need to switch to RPi's to save power?
And if that person can afford the Pi's, they can afford the power, so there's no point quoting power costs.
I didn't say that having resources meant they should be abused. I said that if somebody can afford $150 for a RPi can afford $2.50/mo for power instead of $0.50/mo for power, and that as a result, factoring in lower power use is a poor reason for buying something you don't otherwise need, especially when that power is easily available.
If that power were coming from locally finite source such as solar or wind, or battery, it's a different story.
And if that person can afford the Pi's, they can afford the power, so there's no point quoting power costs.