Personally, I would not want a zoom lens with only a motorized zoom function for photography. YMMV.
I think that depends a lot on how the PZ functionality is implemented. I'm not familiar with Canon's RF PZ systems to date, but I would hope a Canon L lens with PZ would work similarly to Sony's 16-35/4G PZ.
Sony put six of their fastest XD Linear motors into the 16-35G. Four for focusing (2 motors x 2 groups), and 2 for zooming. The result is that the zoom ring has the same feeling and performance as the focus-by-wire system. You can twist the zoom ring really quickly and get really fast zooming, or you can twist it slowly and get slow zooming. The ring is also not a rocker, you turn it the same as you would a mechanically coupled zoom ring, though there are no hard stops AFAIK. Basically, the zoom ring works like the focus by wire MF ring does.
You can also use the zoom rocker on the lens or camera to get smooth constant zooming for video. You can actually set up the lens' zoom responsiveness from within your camera's menu, too. Zooming and AF are both silent, as usual from an XD Linear system.
Another benefit is that the zoom and AF work in concert to maintain perfect focus when zooming, with no delay. It makes the lens appear to be completely parfocal.
As far as zoom lag goes, it's also about the same as what you would experience with a very high end focus by wire system. I suppose if you want to rack the zoom SUPER fast it woujld probably be faster to crank a a mechanically coupled zoom really hard, but the PZ is not slow or laggy.
If Canon implements something similar to this, I think most photographers who are likely to buy an f4L lens would find it very useable. Certainly anyone who wants it for hybrid use will appreciate it.