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.)
This. If someone has bought all their Bitcoins and will never buy any more then they probably shouldn't spend any of them. However if someone is spending 10 BTC and month and buying (by weekly/monthly currency conversion) 20 BTC a month they are getting the best of both worlds.
When someday billions of dollars worth of Bitcoins are held by millions of people one would have to suspend disbelief to think merchants aren't going to seek that out and given attractive deals and the ease of spending those coins people wouldn't do so. In time it will be even easier to accumulate Bitcoins as services spring up that provide innovative ways to bridge the crypto-fiat barrier.