I have yet to see a game where the plot is selected. It appears that the plot costs too much, takes too long and can be more easily foiled than ascension. Am I wrong? Why should OL select the plot over ascension or obsidian shackles?

Some Avatars are stronger then others....so a final fight would be harder to win for the OL player. In addition, depending on the OL player, he may just choose to win by rushing Tamalir. So the plot is Irrelevant anyways.

I guess it all depends on the Overlord player.

Imo, this plot makes the game more exciting, as the lieutenants carry the scrolls and the heroes need to run after them. Also, it makes a lieutenant carrying an item less likely to flee. As for the OL's benefits, the Titan is very close to the Sun stone and as an OL you can choose from different locations to put the scrolls, choosing the closest to your Lieutenants.