The added .85 isn't another .85 in each direction, it's diagonally, everybody knows this. And really? You'd buy a 7.85 but not a 7? Why? Because apple says 7 is unusable but 7.85 is perfect? Have you even held either size tablet before?Because that display size will allows all apps to scale perfectly without distortion or requiring a developer to have to re-code the apps to support a new display size. That's why. That size display has also been leaked as being purchased by apple in an interesting quantity last year (probably for prototypes) and is also the size they have been working to source if you believe current rumors.
That .85" makes a huge difference when compared to a 7" display. That's an extra .85 in EACH direction. If the price is right, I'd consider it. 7" is just too small to really be useful for me.