You are right this definitely is a simple and also fallacious logic. The Bitcoin economy has so far shown the complete opposite of your simple logic!
In real-world economies, you don't normally see that kind of volatility. I suspect that this has to do with a combination of (1) real-world economies' sizes being easier to determine, thanks to taxes and import/export laws and so forth, and (2) the immaturity of the economy making it harder to derive a "fundamental" BTC price from the market price (the significant percentage of Mt. Gox volume coming from Bitcoinica speaks to the degree to which speculation is ruling the exchanges right now).
But the upshot is that whether or not people would choose to save instead of spend if they know their money will appreciate, they can't know that in Bitcoin today. Their money may or may not appreciate on the short-term scale. Maybe tomorrow the price of Bitcoin goes up to $10! Maybe it goes back down to $3! There's no confidence in Bitcoin prices except in those people who "believe it will succeed", who are looking 10 years down the line (too far for Keynesian theories of hoarding to apply, whether or not Keynesian economics accurately describes reality). So the current economic growth doesn't really work as a counterexample when talking about how the Bitcoin economy will behave when it "matures".
It'd be better to talk about the examples of emerging economies in the real world. But I'm not familiar enough with the topic to provide good examples there.