Why do you think every state in US does not run its own currency? How about every county? The alternative is wasteful way to do things, and 50 DACs with their own bitUSD increases the chance 48 of them to end up dead, needlessly.
And let's the bank do the banking.
Actually I think the far more appropriate analogy is BitAsset : DAC :: fiat currency : traditional bank. And just like there are so many banks, there should also be a lot of DACs. In fact I think of DACs primarily as banks with their own value added service on top that makes them special. The currencies can still be limited to a few dominant ones USD, EUR, etc. (or in the DAC world: BitUSD, BitEUR, etc.).
Continuing this analogy, the meta-DAC proposal in my post is analogous to a central bank, like the Federal Reserve. The various DACs can register as "member banks" of the central meta-DAC bank. The withdrawal transactions in my proposal are analogous to a "Fedwire" or equivalent settlement system that allows the DACs to move value from one to the other.
Edit: Oh, and while it is possible for the DACs to operate under a fractional reserve system, which in this case means that they have less BitAsset derivatives (remember that is for example a derivative of the meta-DAC's BitUSD, which is itself a USD derivative; so I like to call the BitAsset derivative on member DACs a second crypto-derivative
) than the BitAssets held in the meta-DAC's reserve, you would always have the choice to not support DACs that implement such policy (and everything is transparent so you can know this) by not depositing your money in their reserves (you can just keep it in the meta-DAC instead).