A developer? Thats awesome.

Mostly it keeps things consistent. With the huge amount of themes out there, and the idea of making a consistent network - you get the idea It would be visual anarchy. Besides that the functions necessitate custom page templates (like the customer area) and again trying to get every theme to play well with that would be a headache.

With that in mind though I was actually thinking of converting it to a plugin anyway, since custom post types are best declared in a plugin. I'd just have to overcome some obstacles as mentioned above.

There were other reasons for going this route as well, but that was the main one.

That makes sense Leo. Switching to a plugin would probably break all the existing sites though, unless you went the route of a plugin and theme combination. Then users could use the default theme (or child) or switch to a theme more to their liking.

The networked nature of Symbiostock is intriguing. You've got an amazing idea growing here. Keeping the visual in line does make sense. Keeping navigation consistent would be important too.