I don't recall anyone ever claiming Diaby was the be all and end all, I think he's attracted support over the years out of sympathy, because he's always put his head down and tried to recover, and because when his game is "on" he's quite majestic to watch.
If anything, the tendency has been to slate him in the sort of terms Klaus does above, accusing him of idiocy, a lack of awareness, ball-dwelling, lackadaisical or clumsy defending etc. Alex Song got the same treatment. It's all a bit over the top. Wilshere is an obvious point of comparison since he has many similar attributes as a midfielder.
I reckon an injury free Diaby would be a very useful player, however at the moment I'd still question whether he'd ever deserve a start. Two or three of our best performers occupy his best positions.