> Why would you want to do that [throttle or shut down a
> booster] even with a liquid fueled booster?
If an incipient major malfunction can be detected early
enough, the booster can be safed before it causes a total
loss of the stack.
Note that the proposed Crew stack has a small escape
rocket on the top of the capsule, as Mercury-thru-Apollo
did (never used, but the USSR had to once). This only
works reliably if what you are blasting away from isn't
still accelerating toward you (or exploding parts at you).
And that won't happen with a liquid fueled booster?