Personally, I don't see the need for David's (and Charlie and Matt's?) forum. However, if that's how they want to approach the whole scenario, then that's their prerogative, of course.
I tend to agree with Mel, and the potential of having one unified site. Look at ExpressoBeans, how it is the epicenter of all print stuff, and there is a lot of good from that, such as the huge number of regular members, databases, etc., that others have mentioned. I'm a fan of the ease to browse and other features of this joint, so I'd say this place has a good potential to be that unified site. Maybe the only thing holding it back is the software, that is not as advanced compared to a site such as ExpressoBeans, but I'm sure it could be integrated into a site like that.
Similar to T, I don't feel like participating across several forums of similar interest, so for the time being, I'll pass on joining David's new site. Also, some sites are easy for me to browse through, and I didn't get that vibe from my brief visit at David's site. It's a personal preference thing, kinda like how Twitter has similar content to Facebook, but I hate Twitter, and only see myself ever going to and participating on Facebook.