This might be (or rather is ) an ignorant inquiry, but I've always been perplexed by this seemingly unflexible good vs evil situation. My issue isn't particulary based on the assigned morality (e.g. orcs are evil just because, and forces of good are so because they are - and I mean in general, as there are surely individual odd storylines).
My problem lies, in TATW terms, in lacking varied combatants. You can either be good and slaughter evil, or command rather bland dark masses against good forces. There are of course evilmen, which add much appreciated diversity, but that's it. And they are far away in the corners of the map.
I was in awe of Battle of Five Armies when I read Hobbit many years ago. Although in retrospection, it looks more like a one-time misunderstanding rather than some common occurence. I think I wished Middle-Earth to be something more like Warhammer Fantasy is.
TL;DR:
The question is, whether there is anything lore-wise that could justify good-on-good warfare?