Considerations for granting read and write privileges to MacHD for "everyone"?
"Everyone" means all users, so yes, there is a risk, but only if someone already got into your system, and at that point they might already be brute-forcing your root password(when they've got it they're almost unstoppable). So as long as nothing bad gets into your system everything's fine and opening up your hard drive for everyone to edit doesn't rise the risk of that happening.