Well the question then is whether you want to exclude grandma and focus on gamers. I sure don't.
This is kind of an academic question right now though (since neither have any good reason to be interested), but an important distinction when it comes to statement of purpose longer term.
In fact on the altcoin observer thread we had an interesting discussion about a billion cell phones mining while on chargers.
The security idea is good. There have been some ideas I've seen to use multisig as a form of decentralized 2FA (both your PC and cell phone would have to sign a transaction for example).
Yeah but you have to figure grandma isn't going to be mining crypto ... someone else is mining with grandma's computer. I'm willing to give her up to get rid of botnets:p
But you are right. It is an academic question. If there was a way of forcing blockchain storage to mine I think that would do wonders because people would find it if botnets were being used.
Maybe someone needs to figure out how to incorporate a proof of consent.