So why buy anything at all with BTC? Better wait, but then wait until when?
Similar reasoning can be applied if you are just to buy BTC and price is $10 more in the morning than it was in the evening. Indirectly you can spend $100 more on the same item. In one day! Again, why buy anything at all then?
The only reasonable behavior is then to just accumulate BTCs
It may depend on how easy it is for you to replenish your wallet so that your spending isn't causing you to deplete your Bitcoin balance.
In the U.S. I can transfer in funds cheaply from my bank using Dwolla. The cost is just $0.25 per transaction.
So I'll have USD funds at an exchange and as I make purchases with bitcoins I'll make a corresponding buy at my exchange so that my net BTC position doesn't decrease.
Additionally, there are people who receive bitcoins as income or wish to convert the increased purchasing power that the rising exchange rate has provided. For them, there are costs and delays to convert that income to fiat. If they can use those funds to pay for their purchases then they don't need to go through the hassle of cashing out.
So not everyone is hesitant to spend their coins.