I first started using Piwik a few years ago when I realized that all our analytics was tied to my Google account, and I didn't want to share that info with my employer. Adding Piwik to the mix meant faster loading since it was hosted on my servers, and passing the data off to the next developer wasn't a problem since the organization owned it all themselves.

I'd recommend using Piwik for sites that you're building for a client so when you hand them the keys they have all the data in place already and there's no need to create new Google accounts just to share the data.