Math Time!
I've been putting this off for a while, but the two of you bring up an interesting point regarding holding B.EXCH.
Ideally, it should not be profitable for people to hold B.EXCH (1 MINE and 1 SELL) long-term. The reason for this is that people could simply buy EXCH and sit on it, contributing nothing to the Derivative - it's modeled after a zero-sum game, after all.
Right, don't hold B.EXCH long time. Buy/Sell short term. Someone who did just this would have gained 8% ROI over the past 30-days buying and selling.
SIMPLE MATH.