It doesn't necessarily matter if it's harder to tax. If the only way to exchange Bitcoins for currency is via face-to-face transactions, 90% of what usefulness it has is eliminated. The remaining use, exchange of a virtual currency for virtual goods and services, is only useful with an easy way to get ahold of a sum of said virtual currency to begin with.

Which is to say, with a sufficiently low volume of BTC being passed around, taxing it becomes as important as making sure you tax cash transactions by roadside fruit vendors or lemonade stands.