You are way wrong unless the grass is way high then it will clump just like the side discharge when it's really high. The side discharge has a more open path to discharge so it can cut a bit higher grass than the Rear discharge before clumping as long as the pattern of mowing is adhered to. With rear discharge one doesn't have to have a pattern. One can mow like a schizophrenic around and around and back and forth and backward, forward and in circle with no concern with making rows where it gathers on two or three or more passes going in the same direction. The rear discharge cuts and recuts more times than the side discharge so the grass is cut into smaller pieces and laid down on the ground under/behind the mower. One can mow up against objects on either side without worrying about chute hitting anything on where the cut discharging grass is going. Ever mow with your grandchild standing near you and knowing something could shoot out the side and hit them? No worry with the rear discharge because nothing shots out of the side, only down on the ground and some what slightly to the rear but directed down and not straight out. For me after many, many years of owning and using both there is no choice to be made, it's rear discharge only if one is made for the machine.
I'm a high grass cutter which is not the Zs best type user but mine does great but lays some slight clumps in real heavy thick grass but not as bad as the side discharge in the same situation unless I keep going in the same direction and work the cut rows building up to go on the edge of the yard where piles of rotting grass isn't an issue and a side discharge won't do this if one likes to do this.
Yes I like rear discharge and hate side discharge and have owned many of both of them and still own one small Z with side discharge because it doesn't come in rear discharge and it's my wife's anyway.