One thing missing from the question is what constitues the SO/SE source code. If it is just an HTTP renderer with database interface and user authentication, then I don't see the problem with open sourcing that. The hosted version can get it's added value from the tools that are used to create the look and feel, administer users perhaps, do automatic backups and any other housekeeping chores. If SE is being aimed at non-IT based companies, they aren't going to know the ins and outs of CSS to set it up properly for the SO engine. I'm sure there are a few other tools that can add to the value of a hosted solution that need not be put into the open source version. This allows the small groups to benefit as well. Who knows, it may prompt someone to suggest to their employer to use the system. Also, open sourcing the SO engine would give you added benefits - localisation for one, especially those non left-to-right-top-to-bottom formatted languages.
Skizz