Economic arguments aside, what this really boils down to is "what sort of city do you want Moncton to be".
NBNYer nicely encapsulated this in a post a couple of pages back.
Moncton will survive without the events centre, but would do so as a miniature version of Detroit, with everyone living in the suburbs and conducting their lives in various power centres and business parks. Almost nobody would venture downtown, and only then if they work there or have appointments necessitating that they go there. That's pretty much the way that I function now. I can go months without taking a walk on Main Street, but I find that rather sad.
A city without a solid core in many ways has no soul. It makes it hard to define what the city is all about and without a definition of what the city is, it makes it difficult to attract new people to move here, especially bright young professionals who could move almost anywhere they want. These are the types of people that we need to attract if we want to remain vibrant.
The choice is ours. Do we want to be a miniature Detroit or a miniature New York City? I don't mean to be grandiose, but I think you get my drift here.......