1. bitcoin value can be foreseen to increase vastly
2. holding bitcoin is foreseeably more profitable than spending bitcoin
The effect (and truthfulness) of that would be dampened by people finding bitcoins risky. After a vast increase in value, some speculators will take profits, and other, more modest investors, will find themselves with too much of their holdings in bitcoins (above 10% in my above example), and thus wanting do diversify out of it. This will dampen the increase in value, or might even reverse it.
I think we witnessed the effect of this when BTCUSD passed 200. We can hope that when Bitcoin matures, investors/speculators will react more quickly, reducing the volatility.
P.S. As a sidenote, some bitcoin clones are trying to rule out the possibility of runaway deflation by implementing a demurrage fee (all holdings gradually leak away to miners, approximating inflation at a controlled rate). I actually consider this a safer bet than Satoshi's, but unfortunately the only credible project trying to do this (freicoin) is operated by a foundation which wants to control and distribute 80% of the initial money supply, and as an opponent of planned economy, I will keep my distance from their project until they've successfully done this or finished trying.
I think many people like the idea that they can store value in the same commodity as they use for trade. With demurrage baked in, the currency will be less useful.