Despite the confrontational nature of the discussion here, this essentially expresses some of my feelings on the matter. For instanced content, as long as the server-side implementation won't slow down development too much, I see no reason to not allow cross-server cooperation so long as it is optional. I honestly have no expectation that any of the proposed downsides of this will happen, and the upsides are many: reducing wait time, letting players from different servers meet, permitting the dev team to design larger instances, knowing that nearly any reasonable number of parties will be frequently available.