reason 1 - Black Knight strategy works when you have a substantial number of them, in order for eviln to be effenctive.
reason 2 - Eviln may not be available straight away and you'll have to make due with whatever you have - this is why trolls are so good.
reason 3 - Trolls can be a good substitute for good units until this tactic can be employed. For example, if you have Eviln available later on, you'll need to make due with what you have, and trolls are exactly that
