I think part of the motivation is to provide standalone apps for people that don't have gmail accounts because they're using hotmail/yahoo/some overseas equivalent.
Like you say, the old experience was great if you use gmail. Now the new split out messages thing is annoying for gmail users, but not tied to a google account. I think their strategy is to break things up into individual lowest common denominator services (like whatsapp), instead of having something that is pretty good at several things, but not the dominant product for any of them.
