Yeah...you can definately just finish all the quests on any difficulty and be able to replay any that u would like as many times as you wish...lol
nad by doing this you can also get more rares and enemy parts,well it's just an idea, and anyways when i did this i got desabers arms( both of them can u belive it)
The only rares I've ever picked up are from Grass Assasins. By this I mean enemy parts which can be changed into weapons.