No one in their right mind is going to use BTC as a currency as long as they expect the value to keep rising. Why spend something now that might be worth 100x as much next year...
You know your cash will be worth less this time next year, thats the incentive to spend it.
well you can, you just replace immediately what you spend, and your were going to spend that anyway, but I do take the point the cost purchase %. However its still cheaper for overseas transactions then interbank rate, or money exchangers