Market symmetry is important. Charging interest to shorts will reduce the demand for shorting while complicating things. The only interest rate I could see making sense is a GLOBAL interest rate charged on all shorts put in place to govern the total issuance of all BitAssets backed by BTS.
Yield on USD is best accomplished by a bond market with fixed terms. BitUSD == checking, bond == CD.
The current lack of demand for shorting at interest is two-fold:
i) USD in the external market has zero interest anyway, so why should shorts pay more? However in other currency markets (especially if financial repression eventually ends) there is a meaningful yield. If bitCurrency holders get zero interest in these markets, we will find it is the demand for currency that will disappear.
ii) We are in a powerful bear phase for crypto and BTS. This has removed the financial incentive for leverage on BTS. This is a cyclical issue, not a permanent one. If BTS enters a strong uptrend there will be no way for shorts to incentivise currency ownership, only through the bond market.
If yield only exists in the bond market, then in either case above, where external interest rates are higher, or where the BTS cycle is more bullish, we will see all transactional use of the currency disappear due to the high opportunity cost.
This still suggests to me that interest is highly desired as a clearing mechanism in the currency market, regardless of the existence of a bond market, assuming we can build it in. Though it is only a recent view I have come to myself.