however using stuff without knowing what it does and refusing quests are not the very best of choises either in a crpg.
this is adding to the injury; people could refuse that quest because they're role-playing. it's a "shady" invitation some uptight/rigtheous characters simply wouldn't want anything to do with it. also, this is a quest involving companion RSP situation, people could "refuse" the quest due to their companions.
taking all quests in CRPG is meta gaming: try to get every secrets and bonus, like a good alignment party taking on an evil alignment quest.
true role-playing, you get some and loose some.
as for the tenebrium issue, it's a dead horse I've been beating. either tenebrium related quests need to be part of main quest, or closing the rift shouldn't be required to finish the game.