If I have $100 (or an asset unconditionally and irreversibly pegged to 100$), what's the point of putting $100 as collateral in the bank to get $50 as a loan and pay interest on it?
Clearly it would be pointless with $100 itself - but an asset is something that presumably you don't want to lose ownership of (i.e. the selling of and buying back would either represent a timing issue or might be expected to be of greater cost due to fees or rising asset values than the paying of interest for the loan).
So I can see the point of using a BitGold asset as collateral for a loan of USD fiat but I could not see the point using a BitUSD asset for that.