Kornel mentioned that he considered it a bug
here but he didn't explain why. I guess that these levers were intended to be dangerous, but the experience (especially combined with enemy spawn delay) made them beneficial.
My point is this: monster levers have 1d3 charges. Why would anyone pull a dangerous lever a second time? (Right, ammo.) I didn't mind the old behavior.
Edit: Wow, that post I linked to is really old. Maybe there's a more recent one...