Because you don't want the risk of centralized collateral? Because you want yield? Because you want to trade it on the decentralized BitShares exchange instead of on other exchanges?
https://tether.to/faqs/
"Tether is the world’s first fiat currency token platform to exist on the Blockchain"
I understand that to mean decentralised. And isn't bitasset's yield close to zero?
You understand incorrectly, but perhaps as intended:
Tether currency is backed 100% by actual fiat currency assets in our reserve account and always maintains a one-to-one ratio with any currency held. For example 1 USD₮ = 1 USD. With almost zero conversion and transfer fees, Tether currency is redeemable for cash at any time.
So it's a token on a blockchain, issued by a centralized entity, and they promise they'll redeem it for fiat. Same as Ripple IOUs, CoinoUSD on NXT, or UIA IOUs on BitShares.
Also bitUSD interest is over 2% right now, significantly higher than my bank is offering on even CDs of any term length.
EDIT: Tether might be a good gateway partner if they'd issue their IOUs directly on our chain also. If they've got the fiat end infrastructure anyway, they might as well use it to its full potential.