I tried to do that for a long time. It's nice until you forget exactly what your email is with say, Company X Industries when trying to reset your password. Was it [email protected]? CXI? CompanyXIndustries? That example is silly but this came up frequently for me. And was super awkward when giving out my email to a CS rep over the phone or "Can you confirm your email address for me?" Me: "something that ends with XP84.cc, I forgot."

Password managers have made this somewhat less painful, at least on the Web, as opposed to "Apps" which refuse to implement the API for using the password -- and username -- stored in your keychain.