In the U.S. I can easily add USD funds to an exchange so that they are ready to be converted to BTCs at any time.   (E.g., sending funds using Mt. Gox using Dwolla, my costs are simply the $0.25 per-transaction fee.)

So I can replenish my stash of bitcoins at about the same time I make a purchase so that my net holdings (number of bitcoins) never decreases.

The reason I might do this is so that I can make purchases where Bitcoin's attributes as a payment network are superior.  For instance, my retailer may give me a discount when I purchase using bitcoins since there are no merchant payment network fees.  Or my supplier will only accept bitcoins due to it being a non-reversible payment method.  Or I wish to send internationally and the delays and/or fees are prohibitive.

Additionally, if I have all my savings in bitcoin, and I want to make a purchase, it costs me money to first convert those savings to fiat (e.g., USDs) and then to spend them.  So being able to spend them directly is advantageous.

The thing is though, it is all a moot point.

If there will be an economic decline because of Bitcoin, then there will be an economic decline because of Bitcoin.  Even if I am told that using bitcoins will cause some economic deflationary spiral, I will still listen to my personal ambition to acquire and use bitcoins versus to avoid them as the result of this prediction of doom.  After a whole bunch of decades watching our currency get devalued to protect us from negative economic cycles, the market is now making the decision to use a different currency -- one that doesn't get devalued.  Like it or not, that's what is happening.