Google's CDN is nice, but nothing's 100% reliable for all users. It occasionally fails to load for me. Other users have had trouble with it due to sloppily-configured networks, and some may prefer to block external scripts by default. It's one more possible point of failure. It would be preferable if Stack Exchange didn't depend on it.
I'm not suggesting that you stop using it, but consider adding a fallback if it fails to load. Something like this: