They do not advertise that feature as much though, and by using email, wouldn't you miss out on at least some of their main social networking selling point?
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 can only speak for myself. I personally user tumblr (only started few months ago) for private ramblings about random stuff that comes in my mind. My tumblr is blocked from search engine and I have not shared the link with anyone. I also blocked out comments and whatever other social media features there might be. I would have made it password protected if they allowed private tumblers to have access to API (you can't post to password protected pages from desktop publishing software AFAIK).
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.
Of course Posterous is just another blogging platform. Have they tried to be anything else? The way you described how you use tumblr does seem to suggest that Posterous is actually a better fit, but it appears that you just found tumblr first and didn't bother to switch.
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?
If I were to take your analogy seriously than it would right to argue that all operating systems are just OS, it makes no difference whether you use windows, OSX, Linux, BSD, solaris.
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.
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.