But this is specifically about the MacBooks, no? Meaning that if you can get the machine to boot it will work as normal, maybe App Store won't work but it shouldn't have much impact on the usability of the machine, right?
You wouldn't get updates and would have to probably blackhole all of the Apple domains. May be enough to not make it resellable (or at least allow the person who bought it to file something with eBay or whoever to get a refund).
Well if the activation lock is still enforced server side you also lose iMessages, FaceTime, Continuity between Apple devices like your iPhone, maybe XCode if it checks for activation during install. If you could get the MacBook to boot it would technically be a usable PC, but it would probably lose a bunch of what makes a MacBook special to begin with.