No, top ribs are not necessary for good shooting. They can, however, make it easier to "come by", and IMO a top rib can help raise that level of skill with a gun. I believe that gun fit is important to achieving our best level of shooting with a particular gun. However, only a minuscule percentage of casual hunters and shooters will go to the trouble to fit their gun to them. Some will try to get the LOP pretty close and leave it at that. Cast and drop, among other things, will be left as is. The shooter will begin to shoot the gun with less than ideal dimensions. As he gets used to the gun's mount he will look down the barrel to check alignment, left to right. In the worst case scenario, the shooter will involuntarily check that alignment as he mounts on a bird, which guarantees a miss behind. The human body being the wonderfully adaptable mechanism that it is, he will make slight adjustments to his mount to get the gun to have the proper alignment when mounted. Muscle memory aids in this, and eventually the shooter will be able to mount the gun in a way that allows him to hit with a reasonable percentage. This will not be the ideal scenario, but it is what I've found to be the case in the majority of casual shooters I've known.
Where the rib comes into play in all this is that it delineates proper eye alignment quickly and readily, much easier than a bare barrel with a bead on the end. I began shooting flying things as a kid, with a S X S with a rib. When I was given a 1100 for my 16th b'day it had a plain barrel. I shot it well, because I was able to shoot so much that I overcame any ill-fit by adapting to it. But, as good as I got with it on doves and quail I never reached my best level of ability until I began shooting guns with top ribs. I not only prefer a barrel with a top rib, I prefer a wide top rib.
Back to the beginning ........ I believe the best shooting one could do with a ribless barrel would be vastly improved by proper gun fit. But, few people will have that done. There's not much chance of me getting "interested" in ribless shotguns. There are too many beautifully made guns with ribs for me to change horses in the middle of the stream.