Years ago I carried all primes because they were sharper than the zooms. With these lenses the only advantage of the prime IMO now is that they are faster. They really have become more of a "specialty" lens.

I did a few comparisons of the RF 24-70mm and the EF 24-70mm L II last night. I would say the IS does matter. The IQ seems to be just a better but not enough that most people would notice and really is not a factor. The specs on the RF 24-70 are misleading, the lens is actually bigger by volume than the EF 24-70. It is fat all the way through, most likely to allow for the IS system. It gives you the feeling that you are carrying a bigger lens.

I took shots at 1/80 at 70mm to get a comparison. In retrospect I should have probably shot slower to get a real comparison for the IS. The EF was the best of the group shot. But hand held with multiple shots the RF did better. You can see if you can tell which is which:
9C0A0619-2 by hdnitehawk01, on Flickr

9C0A0622-2 by hdnitehawk01, on Flickr