Unless it could run a GPU miner it wouldn't be worth the effort
And the people who have really good GPUs usually know what they're doing with their computers I think? Wouldn't they notice the extra heat and noise?
Hard to say. There are a lot of ignorant consumers out there who have no idea what's inside their computer. A lot of people have really fancy cpus and graphics cards, but are totally unaware of it and then only use their computer to browse the internet...
Some people think they have to best gaming computer in the world because they paid 3500$ for it. turns out it has a 5750 and a 980X :p