Yeah, I adhered to the 'only whey PWO' mantra for a long time too. Then I noticed several write ups about different studies concluding that a combination of proteins, fast and slow (something which I previously would use at any time of day other than immediately post workout), actually led to greater muscle growth in the long run. I imagine it more of a combination of factors. Sure, Whey is more anabolic, but casein is more anti-catabolic. Apparantly the chances of the casein slowing the digestion of the whey has very little impact on the overall desired outcome.
While I adamantly focus more on the Hydrolysate forms for ingestion peri-workout, I no longer worry about having a jug o' whey and a seperate jug o' casein/whey (Metabolic Drive) both pearched atop my fridge.