It is a chicken and egg problem. Once you have sufficient liquidity, the spread would decrease and more traders will be attracted to decentralized exchanges. Right now, I think people are complacent. They typically believe they won't be Goxed again. 
Just wait till they get Poloniexxed........then maybe....
the funny thing is that poloniex was hacked already and this made it even more trusted now, so people will not give up on trading there very easily
i think the main problem is that you need the fastest response times possible to attract daytraders. You can't really manage that with a decentralized system at the moment.
Do you envision it for the future where it will be faster to get fiat in and out and to conduct trades?
I think without a centralized entity, it is impossible to have the fast response times you need, at least in the forseeable future. Any cryptocurrency needs validation by multiple instances, otherwise, you could screw the system hard.
A possible solution might be a way of IOU trading, where traders bind their coins in smart contracts that are managed and secured by multiple instances, while the trading itself is managed by a centralized entity, which provides an open, but centrally managed ledger.
Every few minutes, maybe 10 minutes, mabe 30, a snapshot is made, gets approved by the instances mentioned above and coins are being moved based on this snapshot. Ideally, this would function as a "killswitch": in case the central entity is attacked, the decentralized instances can shut down the transactions of coins, minimizing the amount of damage that can be inflicted.
Not to plug too hard, but this may be some usecase for this:
https://bitcointalk.org/index.php?topic=1689947.0
couldn't be done by bot? if two that are trading in a decentralized exchange, which is no different than trading on a forum in the merchant subsection of the alt section for example, and both run a bot for their trading they will get a fast response on their trading activity