Additionally, you don't tend to boulder roll a creep wave of only archers and warriors, so it doesn't come up as much.
Summary; Knockback on Minotaur warriors and archers = no mana for Blade of the Serpent
Lies! I do it all the time because 2 helms = I do what I want! I don't usually go BotS though.
Boulder roll I does 200 damage to 5 minotaurs and for 560 mana. You don't kill the minotaur reinforcements until level 3, and you still wont if they upgrade creep armor, so you'll only kill the archers. I'm not sure but it seems like you net A LOT MORE than 190 mana when you boulder roll a creep wave. I might test this, I'm kind of bored.
OK I tested it real quick and using boulder roll on a creep wave isn't really that good with bots until they upgrade creeps. it's still a good way to kill creeps when killing a tower if you have enough mana.