A lot of what makes them popular can only be found on their site. You don't get any engineering vibe from them, they project a very sensitive image that appeals to the heart. By not visiting their site, you are not participating.
I think the posterous offering is not diluted by email posting, and it would at least appear to me to make more sense if more people on posterous use email posting.
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.
The Posterous switch campaign awhile back was targeted at users like you. That's how they were trying to set themselves apart. It might not have worked well, but the intention is clear.
There is only so much a utility product can do to diversify itself from competitors. A condom is a condom is a condom. As long as it doesn't leak, it works. Would you complain that Trojan's products are not sufficiently different from Durex?
The reason yor analogy doesn't work is because condoms are single purpose, blogging software and OS are not. They can be and has been able to set them apart from one another with features, implementations, ease of use and services.
This ignores quality, convenience, and the user experience. Not all condoms are the same, and how they are deployed by the user makes all the difference in the world.
And don't throw blogger out if you are adding big blogs.