From the monsters description, it seems that most of them should drop some rare materials (to sell or maybe useable to build some rare items) in the final version

This is true, although for quest linked loot it could be something worthless to the player or NPC sovereign.  Like maybe finding a bent sheild with the name of the hero inscribed on it.  In this case you get a shield that has no value but could still be a quest item.

I do like the idea of creatures or humanoids (that might carry their own items/ accumulate loot of things they kill) having random loot..  it makes sense as well as adds another reason to hunt down creature/enemies other then pure exp.

however I suggest that depending on what killed does this killing, in many cases their should be no goodie hut generated.. for example a spider is killer by a orc.. if said spider had loot orc should claim it therefore no goodie hut.. if spider kills orc then a chance that some or all of a potential loot be added to spiders horde would be awesome..

further I hope that in the final version simply killing an npc for exp / loot  has a cost...for example if the npc is of a neutral, allied or friendly faction/race it should have a penalty.. This penalty could be in the form of  reduced penalty (i.e. our sovereign is a ruthless) / increased cost to hire similar npc (this would work well for research/ merchant/social benefit type npc's) or a set number of turns in where npc's (again only works well for research/ merchant/social benefit type) run / try to avoid the killer sovereign's lands.