Agreed... an exchange is more useful the more volume it has.
Although this might not be the right way to do it, I think the goal of the project is to exemplify bitcoin's "distributedness" and minimize the reliance on a single exchange (ie MtGox).
Several exchanges running on the same backend aren't that distributed; there are many scenarios where they would all go down at the same time.
I think the problem isn't distributivity but decentralisation.