It's not so much about which is better: it's about the fact that we need consistency. At least with "leave it to the browser", there is some consistency and users can learn to live with that consistency (e.g. by writing longer posts in a text editor and copy-pasting).
When everyone does it ever so slightly differently, you are lulled into a false sense of security.
I don't mind software sucking, so long as it sucks in a generally consistent way.
Oh, and how many people remember writing responses in a text editor and copying it over to the browser window, so that the back button didn't eat your entire post on a forum when something went wonky with the connection? I don't remember Twitter or Facebook ever losing a submission. Maybe I don't use them enough, but asynchronous submit is a great step forward, and I prefer infinite scroll to many of the paging implementations I have lived with over the years.