Maybe there are no "common" uses just yet because we do not have an easily implemented way to do it. When Ajax first " came out", many said the same things. "great but what do we use it for." now, can you imagine the present day web without it?
I'm failing to see the advantage of reverse http over long polls, even though I am completely sold on the difference that having server push would make...
The only reason I hadn't use xmlhttprequests extensively before it became popular (and renamed Ajax) was because it was too much work, too error prone. The popularization of Ajax lead to solid frameworks and libraries, which fixed that.