At the moment people in BTSX are long term BTSX bulls the BitUSD market hasn't really come here yet. (The main time BitUSD will be heavily bid atm till that market arrives is if people are short term bearish.)
There's a conundrum that if you think BTSX is a relatively safe place to store the value of $ for the next few months that would make you a BTSX bull in which case you'll probably prefer to short BitUSD with your BTSX.
I identified this could be a problem before trading started,
https://bitsharestalk.org/index.php?topic=7416.msg99591#msg99591Possible ideas for solutions
Introduce BitUSD + 10%. It's the same as shorting below the peg but it might look better if BitUSD was trading closer to the peg.
make a trading pair where the people going short aren't actually taking a leveraged position on the future of BTSX If/how this will work I don't know.
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Either way I think it will all work out once BTSX is worth a few billion dollars. Might just be limited general BitAsset demand beforehand but plenty of people willing to short