If you're spinning the shaft to start it, then you'll obviously need something to spin the shaft....the whole thing. In addition to that, the blades will be moving air, so that will increase the torque required.
I assume starter motor are predominantly for compressors (even though turbine is attached from a coomon shaft) ; because after combustion start turbine start to move & carryout combustion one must move compressor such that air is sucked & compressed to combustor. However once process is self-sustained its the opposite; turbine movement causes compressor to move!!