Thanks, Trav, much clearer than I ever am.
However, it wouldn't be so much of a problem if the resource could be bought from those groups at a reasonable price, but being rare resources the cost will be great, and that's if they're willing to sell them to outsiders at all. There's the possibility of stealing them, but they would likely be defended to the point that the methods required to steal them would only be available to groups, which is not a bad thing in itself but doesn't solve the problem.
It's hard to say what makes a resource 'critical to a lot of fun in the game'. i'm one of those people who looks for every purpose for everything and would be disappointed to miss out on anything, whether it's a sword or a balloon animal. I obviously don't expect to actually get everything, but if things are closed off for no particularly good reason then it's like I'm playing a cut down version of the game.