Re: What are these tables?
My 2 cents… Buddypress is built upon WordPress and WordPress MU. If I didn’t like the architecture of WordPress and had concerns with it’s performance to begin with, I probably wouldn’t spend to much time on Buddypress. On the flip side, if I was creating a social compliment to WordPress, I would keep the architecture similar as to take advantage of upgrades, the huge community and extending the system through plugins.
In creating systems in the past, I haven’t designed this way, but I completely understand why Buddypress has made this choice. And I’ll use it. 
