Giving them the benefit of the doubt, I'd say Posterous is smart to stick to building one feature- email posting. This update attempts to drive the same rich content to other sites via email submission? That's a value add. Every feature is a portfolio item with a one-track mind. MT, Blogger and WP all have better plug-ins, widgets, editors, templates, and sidebars. Posterous would be crazy to try to be the newest and greatest blog platform. Instead they are building a the greatest "email submission interpreter"... hoping someone will buy it. Exit and out. Seems straight forward.
My beef is that Blogger and others still have richer email experience in some ways, since you can send to a unique "To" address, thus not limiting your posts to one Blogger account per email address.