I can answer for myself, and rather simply:
I want to move a church / community site off of the .... stuff they have it built with (html editing/ static).
Pinax social template might be a good starting point, but as calendars, docs, and groups (as well as logins) would all be (and currently are) used as google apps / google docs, and (up to a point) the hosting would be no / low cost, the motivation to make this port exist on GAE is (at least initially) very strong.
In any case - each application will have it's motivations for where / how to host, and pinax is not a canned solution (i.e. not a drupal, or a plone), but rather a framework to build applications from, so the hosting choices are bound to be varied, and for varied reasons.
Regards,
- Yarko