I for one, don't think the inability to turn off/on Ooma VM (as a user setting) is a big deal. If one prefers own answering machine, make Ooma ring longer and have own answering machine pick up. A few occasional VMs left on Ooma despite setup (during network outage, power outage, etc.) isn't that much hassle to put up with. At least you have your VM somewhere.
As for not wanting VM at all, set your VM to pick up at 59 seconds (that's almost 10 rings); if that doesn't work (if people still want to leave you messages, you might want to reconsider who you hand out your number to

).
If you get too many messages even with pick up at 59 seconds, go back to option 1, have your answering machine pick up before Ooma (get an answering machine that would initiate outgoing message "VM full" & hang up; fill up your VM box beforehand). Turn on Ooma VM by turning off answering machine; turn off Ooma VM by turning on answering machine. You'd still get a few occasional VMs left on Ooma despite setup (during network outage, power outage, etc.).
If one hates VM so very much, just call Ooma and have them turn off Ooma VM forever. If there is hate for VM, what's the problem with parting with it for good?
Like I said, I don't think this is a big deal. But obviously others differ. I am pretty sure I am in the overwhelming majority on this one.

If not (if I am wrong), and Ooma wants to accommodate, this is seemingly easy to do but no one can be certain (only Ooma would know). I know that Ooma creates and wipes VM boxes all the time (when personal devices are created, number assigned and vice versa) but what kind of operational challenges would checking VM flag to see if call should be directed to VM or "VM full" message bring about? Even if this is easy, wouldn't Ooma want to add this to Premier vs Free Serive?
Bottom line is, IMHO, this feature is a nice to have vs a "Need."