People need to be able to borrow any asset from others and pay interest. Then they can pursue whatever trading strategy they want. No one knows what can happen in 30 days. If you're trading with a long-term strategy, then you don't care about a 30 day movement. But if they're forced to cover every 30 days, then many people will be unwilling to trade.
Lending/borrowing markets are planned for BTS, right? BTS should try to emulate Bitfinex and other successful trading platforms as much as possible. They have already done the research and gotten the feedback on what traders want.