Yes, Divine Divinity did have certain rare items which had completely random modifiers, but appeared in the same place each time. That could prompt saving and reloading. Respawning enemies would not change that.
One solution is to have enemies give random drops which had a chance to be extremely good. Another is to have the extremely good items with fixed stats in the same location each time. The second solution would make it more predictable. Random items are more for a Diablo-like game where item fever is part of the point, I don't think Original Sin will be an "item fever" game.
One solution for accidentally killing special quest monsters might be to only have them spawn if you take the quest. Another is to consider the conditions met if you kill them before getting the quest in the first place.
The Yendorial Tales example is an argument against poor use of gating progress - poor game design, not an general argument against no-respawns. gates can be designed more intelligently with multiple ways around them. In this game for example, Larian said you can kill everyone and still be able to complete the main quest.
