So what you're saying is, it doesn't do something that you could already do using something else?
I've seen this logic applied to many software projects and while there are many justifications, I don't see the point of duplicating functionality unless it's done in a way that is substantially better or more elegant that the original method. I doubt that the developers at Posterous (who I have great respect for) can justify the resources to write a better tool for assembling multimedia messages than the one Apple provides with the iPhone, the Mail app.
There is certainly room for varying opinions on this, but given Posterous's design philosophy so far, I think duplicating functionality would be non-consistent with the work they have already done.