I'm not really sure I understand the point of the poll. Human or AI should only cast this spell when there are a lot of enemy units (or a very powerful enemy) that is either in your territory or within the treants' short movement range. Once cast, all treants should move quickly to attack. So, I guess the answer should be "no". I think a better question would be: when should the AI cast this spell?
1. DoW'd by powerful civ.
2. Many (or very powerful) enemy units within range of owned forests.
I'm assuming #1 is current method, because I can useually kill Ljoslfar by: 1) DoW, 2) watch the Ljos power graph go through the roof, 3) wait till power graph returns to normal, 4) begin conquest. (You don't even need to move your units far away -- just leave outside border and you seem to be safe.)
(Btw, I really like this worldspell. I almost never use it, but I think of it as an option of last resort. Sort of like nuclear weapons.)