The thickness of a case can often double overall size and weight in your hand. This that isn't going to change, probably ever.
Once the early adopters flash their iPad airs around with no case (or an extremely thin mag cover), an ipad 3 is going to look fat and heavy. A nexus 7 is going to look bulky and old. Soon it will be a basic expectation that tablets are light as feather computers with no perceivable thickness. Tablets that don't come in this form factor will be seen as clunky relics of the past.
The best way to tell if an apple product will do well, is to ask your self: "Once this product gets wide spread adoption, will this product make its predecesor seem, old, clunky and/or unrefined". From a tactile and visual stand point a iPad air will do this.
Full disclosure: despite my argument, I hate cases on my devices and refuse to use them aside from a thin tablet screen cover.
There is a severe flaw in what you're saying - Apple didn't design the iPad 1, iPad 2 or iPad 3 with a full case in mind, in spite of the audience using them lots.
Yes I realise they make the smart case. I think of this like the bumper - offering something demanded, but the core product was designed without it. They also introduced it significantly after the iPad 2.