One point of reference: World of Warcraft and other games (apparently Windows 10 too) have used P2P (torrent-like) patch systems in the past, where after you d

One point of reference: World of Warcraft and other games (apparently Windows 10 too) have used P2P (torrent-like) patch systems in the past, where after you downloaded a content update, you also distribute it to other users. AFAIK this was opt-out rather than opt-in. I think that falls under the same category of using the users' computer for an extraordinary purpose.