Earlier quoted context omitted.
I don't want my stream filled with junk the way my Facebook stream is. Me either, but - IMO - the answer to this isn't to leave out the "write API," but rather the answer is to give users better filters and the ability to define their stream to meet their own desires. Of course doing so with a simple and intuitive UI is the tricky part, but hey... those G+ engineers make a lot of money, they should be able to come up…
> they should be able to come up with something, no? no! :) Probably the hardest thing in social media is noise filtering via a UI (it's hard for Google, it's hard for Facebook, it's just a hard problem). For my own (app) selfish reasons, I'd like to see a write api for comments first as that doesn't cause issues on the same scale as writing posts to the stream. Could be a nice place to start.
Ad hoc groupings of people in your Lists and ad hoc set operations on your current groups (school intersection friends, union of all groups filtered where profession = X or Y, etc.) is something a write api would let a budding entrepreneur develop an app around and potentially make a lot of money.