Thanks for asking for clarification; it's not really relevant to this discussion. I'm glad MixPanel has this new product, I'm sure website owners will find it useful. My primary intent in posting was to let MixPanel know that people like me can't even click 'See it in Action' because JavaScript processing stops after the first error (counting on MixPanel loading).
I'm all for apps using the data I give them to build usage patterns as a part of improving their service; that's an unavoidable part of the value they provide. Cross-site collectors a-la Google, Facebook, Adobe, MixPanel, etc. eliminate a barrier to a centralized understanding of everything I do online.
Edit: TL;DR - Pikwik yes, MixPanel no. And degrade gracefully if your external JavaScript doesn't load, especially analytics.