Supporter vs Pay for Blog


More Newbee confusiuon...

I'm using BuddyPress & understand that the Pay for Blog plugin is not for Buddypress.

However, I do want to charge members to blog. Maybe other things too. If I only charge for a Premium upgrade for Themes and plugins, then
A. Where do all these premium themes for BP come from?
B. There are not many regular plugins for BP users, lots for WP-MU, but your list of 6 plugins is not worth that much, are they? What am I missing?
Those regular plugins known to work WPMU/BuddyPress include:
1.  AddThis Social Bookmarking plugin
2.  Akismet
3.  Audio and video comments
4.  Greet Box
5.  Subscribe To Comments
6.  Threaded comments

"So what does this plugin do exactly? Well, essentially it allows you to offer and charge for premium features or use of your WPMU or BuddyPress site - collecting either subscriptions or single payments in return."

So it says I can "or use of your WPMU or BuddyPress site". To do this do I use a "whole new signup form" to do a subscribe at signup?

The Pay to Blog seems much more clear cut and has good features.
Why would PB users not want Pay to Blog functionality?

Kind regards,