I know this is not how or why most people use tumblr. But thats ok. I use it only the way I like it.
I also have a "public" self hosted wordpress blog which I share with my friends and family and open to search engine.
posterous, to me, is just yet another blogging platform. Just like I don't feel like I need to try out every single new things comes out every week, I don't feel like posterous really sets them apart is such a way from all other existing blogging platform for me to even bother.