I agree that standart SMP not strictly needed. I think that Asymmetric Multiprocessing was there before SMP and it worked.
From my poit of view is good to use any system of multiprocessing which means less work for developers and preserve most of compatibility.
And regarding the 64-bit, what advantage it will have to switch to 64-bit only? MorphOS have no many applications ( if any ) which runs out of 32-bit address space. I think we don't need 64-bit from technical point of view. Maybe a little advantage can be, if 64-bit code runs faster than 32-code on 64-bit CPUs, but I am not sure if it is true.
Even windows and linux not break backward 32-bit compatibility. And G5s are not 64-bit only, they can execute 32-bit instructions and have support for 32-bit OS.
AmigaOS3: Amiga 1200
AmigaOS4: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad