Well, the upside would be that the economy might be less stagnant than it is right now. I mean, if you do an item search, most < 40 gear is not even available on my server. People just wing it in whatever they can find at low/mid levels. With more buyers, maybe things will sell faster and at least some people will bother with crafting and listing those items again.

The downside is that people will be able to transfer wealth across servers. But I'm not sure this is really a downside. What is the net negative impact of people moving their own assets anyway?