In my case, I would keep different species in different cages, so being able to remove/modify one cage w/o disturbing the others was a real plus. In other words, I like racking individual cages as opposed to stacking, so the slant top was of no concern to me.
Also, on thin plastic cages with sliding doors, stacking can cause problems with the doors on the lower tier of the stack. Then the cage has to be strengthened around the doors. Racking eliminates the need, resulting in very lightweight cages for the larger cages. Meaning also that the cages do not need lips around the door track for strength, meaning when you wash them outside with a hose (like for large boa cages), water doesn't get trapped inside.
And I like that the racks have wheels to make moving easy.
-----
Mark