Reason for this is jet engines require time to come up to power, if you do not have them stablised the response time in the event of a go around(Idle thrust to go around power in the event of a "glide app") would be unacceptable at least with decision heights that are normal/required for all wx airline transport ops.
Used to be this way on older designs, however....if one has three shaft Rollers installed, these engines spool-up mighty fast.
Example.
Lockheed during certification flying for the L1011, a flight idle approach to 200 feet AGL was tried several times, yet the engines spooled-up just fine for a go-around.
And yes, I know the guys that 'done it.'
Cannot be used in commercial service, of course, but demonstrates the adaptability of the Roller's three shaft design.
RollsRoyce...builds
good turbine engines
A superb design...quiet, too.