I'm only in the factional hubs so I cannot give you insight into the others, however, I always use their discords for help with phasing or for my OOC connecting. Using the in-game guild isn't a great help anymore I've found.
For instancing, yes it does matter to be connected to the rest of the community. However, Discord has become the best way to do that, to the point where I feel like the in-game hubs have become a little outdated. However, this is personal opinion and I am certain that others will disagree.