This is well understood to be because Dwolla pulled the rug out from underneath them.  Once ACH comes online in a few days, CampBX will be back in line with the "non-Gox" USD exchanges...that is, exchanges that you can actually pull USD out of.

And I trade on Bitstamp not because I'm planning to pull my USD out daily or something, but that I know at SOME POINT I will want to pull some USD out, and I would really like to have that option.

Plus, in the event Mt. Gox ever does open up USD transfers, although why bother at this point -- but if they did, then arbitrage would immediately send the Gox price down and raise up all other USD liquid exchanges to level the prices out.