All credentials under one master password? Single point of failure. You could use more passwords, but we are heading back to square one then.
Next you have to decide where to make your passwords accessible for yourself. Do you also want them on the phone? Because if you don't, it can get kinda inconvenient. On the other hand, I'm quite positive my phone has more exploitable security issues than my laptop. Same for all other devices you might use. What about devices that are shared by other people? You lock yourself out of the things for which you've opted to use the password manager, or you expose it to the security issues on the said devices.
I'd rather just have less stupid passwords to begin with. Why do most stores require an account for me to place an order? Not for my security. Forums, boards, other places.. anonymous posting without accounts works just fine, and there are ways to create a persistent identity for those who want it, without requiring it from everybody. Yet most "social" sites require accounts. Mostly not for my security.