Sharpness is relative to the distance you view the content at and the real world size of the screen.
eg - on a TV you only perceive full HD up to a certain distance from your plasma after which the detail becomes imperceivable.
On the smaller ipad Id rather have the CPU driving snappier apps and grunty creative stuff than driving more pixels just to read my email.
Retina a little overkill except on the full size iPad?
With that logic, the iPhone and iPod don't need Retina.