Re: What are these tables?
It really isn’t that hard to believe since the plugin is creating 3 tables for every user when WPMU creates 8 tables for every single blog right now. It makes perfect sense from a scaling point of view because there are already solutions to split the information from WPMU across multiple databases and something in the works for scaling BP information as well.
There are multiple installs of WPMU right now having to use multiple servers to run their installations and a large BP site might have to do the same if it is busy enough. Having the data in just a few tables doesn’t allow scaling and puts limits on something that doesn’t have to have any. 50,000 users information in a couple of tables is not the best solution. I would rather have those 50,000 users spread across 4000 databases so that you are scanning for 12.5 users per database. Even better would be those 50,000 users over 256 databases. With 4000+ databases you can have millions of users 
