As depicted in the whitepaper, BitUSD will be destroyed over time due to fees. That means there exists excess XTS backing BitUSD. The theory states that if the backing XTS is more than the original amount, BitUSD will trade with a premium over USD, effectively paying dividend to the BitUSD holder. This also serves as the motivation to hold BitUSD.
Here is the concern. We need a channel to connect the excess XTS to the existing BitUSD, in order for the market to perceive that a BitUSD indeed is more valuable than a USD. In other words, the excess XTS will somehow need to be cashed out. Otherwise, if it stays within the system for ever, then it only provide more security for holding BitUSD against short squeeze. Other than that, it is no more than a notion, and as a results BitUSD price stays with USD, which voids the motivation for holding BitUSD.