We kind of do that, unless we're talking about exceptional talents like Messi, Busi, Iniesta and others. And those guys delivered right away almost.
They were promoted though, and given regular minutes to prove their worth, although they were exceptional talents as you said so easier to give them a shot.
Actually, Busi wasn't counted as an exceptional talent by many people. When he was brought in, he made some wrong decisions at times and people got mad at Pep for not playing Yaya over him. Worked out quite well in the end I must say.
Could have got Reus over Neymar. Or we could have gone for a well established, experienced option instead of bringing Pique back.
Anyhow, I don't think GKs should be compared to outfield players. Even a phenomenon like Neuer at Schalke and De Gea who was rated very highly in Spain even before his move to Man Utd went on to make some really, really bad mistakes.
I agree with Alik above and that's what I've said since day 1. You either give him consistent minutes or you loan him out like Chelsea did with Courtois. You can't expect him to be WC and don't make a single mistake when he got irregular playing time and faces more difficult opposition in the Champions League that way.