As far as I understand, the scenario to be feared would happen, if the following chain of events happened:
1. bitcoin value can be foreseen to increase vastly
2. holding bitcoin is foreseeably more profitable than spending bitcoin
3. a point is reached where creation of new bitcoins has almost stopped
4. additionally, a point is reached where few transactions occur with bitcoins (holding is profitable, after all, so everyone expects their money's value to increase)
5. transaction fees are not sufficient for miners to make a profit
6. miners start requiring higher transaction fees
7. transactions become more expensive to make
8. fewer transactions are made
9. miners start leaving
10. with less miners, there is less security (easier to mount attacks)
11. attacks start succeeding
...and at this point, either value drops and it becomes reasonable to circulate bitcoin instead of holding it, or attacks succeed to such a degree that confidence in the Bitcoin system is lost.
In an ideal world, people realize the possibility of such a chain of events, and avoid holding unreasonably large amounts of non-circulating BTC. Circulating BTC is not harmful to network health, as it can be "taxed" by transaction fees and offers incentive for miners to secure the block chain. If the block chain is sufficiently secured, attacks will not sufficiently succeed, and confidence in the system is not lost.
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.