You are right to point out that not all players should be world class in a club. But we already have those. Pedro, Biscuits, Adriano and Keita (whom I used to like, but I feel is getting old by now) As good as Pedro may be, he is not truly world if we are being honest here, so goes for Biscuits.
Afellay, I fear, cannot play the role of a squad player in any meanigful way. We should have sold him in the summer for a premium (thank God we bought him cheap) and bring a younger talent that may deliver the goods without breaking the bank.
As for Aguero sitting on our bench...You gotta be kidding me. Aguero is far ahead of anything we have in our forward line, except of course Leo Messi. He would be an automatic starter.
I intentionally sound extremely harsh on some players, and with Barcelona in general (after 09/10 I said that I was bitterly disappointed due to the Inter defeat) because one thing that I realised about football, is that the managing staff, the board, as well as the fans should never be fully happy and content. You should always look for your weaknesses, what you have done wrong, what can be improved. Even if there is little that can rationally be criticized, you should even maybe make up something out of nothing, so that permanent agitation persists. Otherwise you have stagnation/regression, two conditions that are paradoxically synonymous.