Well it does indeed get worse with Mok in the equation, but the point still remains, that the longer the quest drags, the harder it is to keep OL cards flowing to your hand, so that you lose steam to compete once you get close to the end of the quest.
I don't know, that's my experience of OL cards. I tend to play my best cards relatively conservatively, so they hit their maximal effect, but that's only if there is no discard on the table - otherwise I tend to play them before they get removed from my hand. Other than that I tend not to hold my plays too much, because the smallest interaction I can have with the heroes can give me more options. Basically, not playing anything only leaves you with your monsters and that's not getting you very far in general. plot cards are also very situational for the most parts and I´m guessing saving OL cards in your hand goes in pair with not wasting threat/fortune before it's "time".
Psychologically speaking you need OL cards just so your hero players hold their plays in case you drop something on them. It has the disadvantage of making them be more careful, but they might still lose precious actions/turns just playing safe while you´re just buying time to do whatever you had in mind.
Edited by Indalecio