Build websites on Atrium?

I wonder if it would be feasible to build websites using Open Atrium instead of straight Drupal. A good example is if you wanted a job search site integrated with project management, similar to oDesk. There are two approaches: (1) build the job-search site on Drupal, and the project-management bit on Atrium; and (2) Build the whole shebang on Atrium. Here are the pros/cons I see:

Drupal + Atrium:

Pros: Separated from Atrium's dev-cycle. You're not at the mercy of which modules/version it allows, you don't have to worry about changes you've made come Atrium upgrade

Cons: You'd have to connect the two somehow... Single Sign On? Shared user tables? What if you wanted a rich set of profile info (rating users, content_profile, etc)

Just Atrium

Pros: Everything's integrated. You wouldn't have to connect data or sign-ons from two systems

Cons: You'd have to worry about customizations you've made, which modules/versions are compatible with Atrium features, what you can/can't disable.

I think the Drupal + Atrium combo sounds more attractive, but I wonder if anyone has experience (woes or otherwise) connecting two Drupal instances together by more than simply the login session