Botany diploma fight

[Metathron]

Of the three Lecture hall diplomas only this one requires you to have a certain amount of leadership (min. 16,700) and it takes away your army and replaces it with ents, thorns, etc. against Dumbledore's (or whatever) army of red dragons, catapults and fire spiders. Transparent much?

1. Why is your army replaced in this case but not in the other two?
2. What is the deal with the insane leadership requirements? My Amelie, with a leadership of 21,500 cannot control two of the stacks, one of which has a leadership of 62,700 so "coming back later" is not an option (not that I'd want to).

So, what the heck is so good about the +10 attack and defense to plants and ents that the diploma grants to warrant all these special modifications?