For what, for make yet more advantage for the players who play to play?!
I see your point. It would help in cases where you are waiting to get past content but don't have that 1 specific champ you need, but I think it would be abused and every account that spends would end up with all the same champs. As much as they originally charged for vision it would take the average f2p player all year just to have units for that 1 champ with none left for the content they need to complete.
My point is that certain champs like Aegon is the only champ that can really help you in content like LOL and AOL. So how is it unfair?