Actually, it was usually Holzman who soloed, while Irving was primarily playing grooves and colours.
Kei Akagi was probably the most talented of the late period keyboardists (I haven't heard enough of the band when Joey DeFrancesco was there to make a judgement on his contribution) but they were pretty much all there primarily to weave textures around the soloists (usually Miles, Garrett & Foley).
Pity the group with Johnson didn't have longer. He sounded promising.
As for the 70s band, Miles seemed more concerned with rhythm than anything melodic or harmonically complex. With Reggie Lucas chugging away there seems little that an additional instrument could add although Miles' own rudimentary organ stabs worked surprisingly well most of the time. A virtuoso would probably have just got in the way.