I'd say it's more like a tradeoff than defeating the purpose.

And not as much of a tradeoff if a lot of nearby visitors have also visited the same site, or users visit it frequently.

I think that depends on user. Some use CDNs to improve performance via already cached files, others to offload traffic and use the actual CDN performance from Geo balancing.

Personally I don't think a user will have anything cached other than jQuery from Google. Which in this case I removed to follow the rule of eating own dogfood.

It kinda depends on statistics; I'd like to see some figures about CDN cache hits like that. I know the theory is sound, but I also gathered that one usually only gets 20% cache hits on websites with caching enabled. or something.