re: Being able to shut off the engine??
The issue comes when you ARE off the ground, at a safe elevation, and THEN you have a malfunction.
It's a rather safer idea to be able to switch everything off, seperate things, and parachute down, than being strapped to something with the power to burn all the way to the edge of space, without an "OFF" switch.
What if the stabilisation system fails and you stop pointing "up"? It would be a damn good idea to switch off and let go, rather than being pressid into your seat for a 10g piledrive towards terra very firma...