I think there is a third way in the middle between the two ways krama described: in addition to the rares for general content, krama mentioned some examples, keep rares that are good for faction wars. You don't have to level or equip them right now, just keep them for later use in your vault. At some point later on in the game you probably want to do the faction wars, and some rares have abilities that are, well ... rare inside of their faction.
For example Fleshmonger. There are exactly two champs from his faction Skinwalkers that can put the big decrease def debuff on all enemies: the void legendary Khoronar and Fleshmonger.
Another example is Geargrinder. He is a reviver. You don't need him for general content, because you allready have Scyl of the Draces, who is a far better champ with revive ability. But in his faction Ogryn Tribes, he is the only real reviver. The epic champ Uugo has a revive ability, but it's conditional and works only, if all other champs in the team are dead, not to mention that you don't have Uugo at your account. So Geargrinder could be usefull for your Ogryn faction war team.
Typical abilities you want to have in faction wars are decrease def on all enemies, heal, revive and turnmeter reduce.
P.S.
I thought as far as rares go that I would keep Apothacary and those you
listed although I always thought Master Butcher was trash
Master Butcher is not only great for Bommal, but also for Magma Dragon. Master Butcher has a two turn provoke with 100% chance to land on his A2.