A BTC holder that believes the btc price would go up he would not spend his btc on a service that is currently being valued in dollars.
I see your point now. Maybe we'll think about giving both options.
Yet, not every BTC/USD bullish is necessarily BTC/ZEN bullish.
On a second thought, Zencoin is a commodity, and the sale is not intended for assets speculations. So maybe we'll keep it fiat.