Suppose I said to you "hey, I'm going to run a new 'direct' pass-through for ASICMiner, so you can now get shares of ASICMiner directly from Friedcat, or you can get pass-through shares directly from me". Why on Earth would you buy them from me? Suppose someone now said "hey, did you know ASICMiner is finally going to be listed on an exchange again?" Then the question becomes even more pressing: why on Earth would you buy them from me instead?
Ah, that's actually a great example.
First off, I think the broader question is "what are potential reasons to justify a second instance" rather than focusing on the need of two exchanges. Generalized I would answer this with "whenever there is a difference between the two instances, there might be a justification". This also applies to direct shares, especially when the operator fulfills some roles of an exchange.
As mentioned before, the current form of this PT is not the one I intended to create in the first place, but the result of the LTCG/BTCT shutdown. I decided to continue simply (but not only) because there were shareholders left who didn't convert to CM. From my point of view that's justification enough, even if you think there is "no need to". I'm not the one who makes that call, but shareholders do, based on their own reasoning and the given circumstances.
It's also worth to mention that this PT is tradable in the sense that I do process transfers from one user to another, which CM as direct share provider never did and never will with the note that users should wait for CipherTrade. By the way, I'm happy about any meaningful suggestion, if you like to contribute.