Re: All members from first registration auto ability to contribute to default blog?
If you just want users to be able to submit blog posts, why not use regular WP instead of WPMU? In the WP settings, set the user’s default role to be contributor or something higher than subscriber. Then they can access the backend and write blog posts on the blog.
Note that this functionality doesn’t really have anything to do with BuddyPress.
The only reason to run WPMU is if you want your users to each have their own blog. If you don’t want to offer that, don’t run WPMU. 
BuddyPress just provides the social networking functionality (activity streams/updates, forums, groups, etc.).
As for Burt’s plugin, I think it might not longer be necessary in most cases now that BP runs on WP Single and not only on WPMU. I’m probably mistaken though.
