What value does the decentralized back-end provide for your average consumer who only interacts with bitcoin through the front-end? It's true that cash-like digital transactions are still possible through bitcoin, but the utility of bitcoin as "money I can use in exchange for services" hinges on the fact that these centralized 3rd parties allow individuals at the company's discretion to spend their bitcoin.
anybody can verify that coin base acts as it should.
Tell that to those who were swindled by mt.gox and the long line of shady/incompetent bitcoin players that have screwed countless bitcoin users.