Credit cards are the best place for an emergency fund. You still all your cash in US-Treasuries, gaining 5+ % interest with no federal tax, and as secure as anything in the world. Then if an emergency comes up, use your credit cards, and you have 30 days to pull money out and pay for it, without losing any of your interest.
Unless your emergency comes during an economic downturn or credit crunch, or the issuer gets wind that you've lost your job, or really anything that lets them change the terms underneath you. And they'e inevitably reserved the right to do so.
It's rare to find a revolving credit line that you can actually know will be there when an emergency comes -- canceled cards, reduced limits, suddenly prohibited cash advances, exploitative rate changes, etc are all things that do happen during tighter economic times and just haven't been widespread in the last decade or so because money was cheap. Be careful not to be conditioned strictly on those freewheeling days that are now ending!
You can live the way you describe, and many of us have gone through periods of needing to rationalize it and do so, but with a clear head it's not something I would characterize as "the best blace for an emergency fund" because you just can't really trust it.
