It's annoying when you've just achieved something great (take over Equestria by banishing its princess, locking away all those who pose a threat in an unescapable place, ignoring five little nagging voices at the back of your head) and there is no one who is not scared of you to congratulate you on the matter.
Still, being an all-powerfull evil thing, you can get anyone really to do what you want, without any plan B whatsoever.

So when things start happening that go so, very much against what you planned, a plan B may be just the thing you need.

Especially when the battles are fair and you don't have the advantage of being able to outnumber your rival.