Items seem pretty worthless unless you're using something like a revive. Most of the time their effect doesn't outweigh the cost of wasting a turn to use the item (antidotes and the like).
On top of that, there's an achievement for using items but it would literally just be for the achievement if I were to ever use any items. There were a couple of times during the adventure map where the multi-target stun items were useful but the scenarios were far and few between.
If items were to not waste a monster's turn but be limited to 1 item per monster turn then they would become useful and maybe even desirable. I would think that some items (like revives) might want to still retain the turn cost but I don't see why the majority of items couldn't be used without consuming the monster's turn.