Part of this being a roleplaying mod is that some of the sandbox elements are thrown under the rug. Yeah, minor factions are great and all, but they're all bad and want to kill you, and there's no point in helping them with anything, especially the ones that spawn armies (exception being Noldor, because SOME people are pansy elf lovers). The only point in befriending them is if they spawn in your kingdom's area and you don't want your villages being wretchedly poor because they can never get back and forth from their destination.
They aren't treated like a separate kingdom upon entering the field with an army, kind of like how the Black Knight invasion works in the Native Expansion mod, so they can never hold land. It would be pretty nice if these invading armies would actively try to ruin everybody's day by sieging fiefs and holding them, and having their own kings and lords, but that would take effort I'm not sure people are willing to put in. Besides, you're supposed to be the good guy. Kind of lame, but that's just the nature of the beast.