What you say makes sense, but here's why I'm interested in using the Other Machines option for restricting . . . sometimes the kids have their friends over; I'd just as soon have their devices go off at the 9:30 shut down time as well. So, instead of having to look up their MAC address and enter a separate policy, I'd just like to say "Mom's computer never gets shut down (Never * Block All Access), all Other Machines * Block All Access * at 9:30". Two policies - done. But, again, when I enable these policies as stated, at 9:30 everyone gets blocked.
Currently, I'm basically doing what you say - I have 6 policies, one for each of the kids' devices to shut down at 9:30,which seems to work - just wishing it was more general.