You’re very welcome to chime in here, and now is a good time as @tad and I are getting set up to renew efforts towards community sites.
GA is an option and it comes with the Google Apps that communities already use if they have email through us. I agree that being able to see trends across communities is valuable.
I do not think the majority of sites use analytics at all though. Piwik gives us a chance to teach important skills that a 3rd party hosted solution does not. The Mozilla internal teams have more than enough work to go around and so farming out what can be offloaded makes sense. Good tools should allow us to grab the same stats, so we should still be able to compare the numbers. I would hope that the metrics teams would be able to help us out no matter what tool a community decides to use.
I have come around to the belief that we have so many communities with different needs that we need to be prepared for a diversity of tools for similar tasks. If for no other reason than better tools come along, the existing tools don’t always stay at the top of the class, and some communities will switch to them sooner than others, while established communities keep to the old tools as they’re ingrained.
You raise very good points, and GA is available to communities if they like. I think what we need more than to pick a tool is to learn what sort of data we should be collecting and making sure to collect it in a way that can be compared tool agnostic - that way the data is always useful even if communities start with Piwik and choose to move to Google, or vice versa.