That’s a good question.
I did some research as well, and planning to give BuddyPress a try soon. I just finished going WPMU from WP and finding it very interesting. I love it so far.
I think the answers that you and I are looking for would depend on community system.
To simply put, you won’t have problems with WP if you’re not planning to create a huge community-based websites. A portfolio website, or any kind of your personal or single-topic website wouldn’t have too much problem with it. WP actually has better participations from people around the world than WPMU. And with bbPress, you can setup your community with ease. The main reason that you would want to use bbPress with WP is the user integration. People who registered at your blog can go to forum and post without creating another login, or vise versa.
As you know, WPMU can have a single unified user DB so any users who registered from maindomain.com, site36.maindomain.com and hax0r.maindomain.com can post any other websites as long as they’re within your WPMU system.
BuddyPress is pushing that idea further, bringing everyone on your WPMU system, using the famous Facebook-like wall system, but not limited to.
Let’s say your maindomain.com is about cars, but the website that you gave out to your friend is area51.maindomain.com which is about UFOs. You have two separate websites of two separate interests. And let’s say each website has their own bbPress forum, then odds are that your car people has no chance of meeting UFO people, because they’re not sharing the same interest, therefore no reason to hop over to the other website’s forum. That doesn’t mean the people who like cars don’t have interest in UFOs, but since it’s two different interests that members wouldn’t go back and forth just to share their everyday discussion. Many people are used to the people on their favorite forum, so they stick to the forum.
With BuddyPress, you can bring these people into one place. Anyone can start a discussion about anything. And with BuddyPress, there’s enhanced profile, friend connection and private message systems that bbPress would require plugins for.
bbPress is like a meeting room, people gathered for the same interest.
BuddyPress is like a lobby, where everyone can meet and make connections.
If you want a lobby for everyone to come together, then BuddyPress will help you just that.
If forum system works right and if you’re not interested creating a lobby for everyone, that’s cool too. It’s all about how you want to manage your community.
And where did you hear about the WP and WPMU integration? That would be a great news for WPMU users since we’ll have more community participating for catching bugs and making plugins.