Helps if you need to fall prone on purpose for a round if there's no other cover. One maneuver vs. two to drop and pop up makes it a much more viable tactic.

Knockdown is not that bad and many talents are situational. In this case, depending on the initiative it could grant a boost due to multiple allies attacking a prone target and then force the adversary to waste its maneuver on getting up. Not great, but not bad.

i think it’s cost is ok. Not every talent on every tree is a winner. It’s unfortunate for most trees that knockdown costs 20XP, but for many groups that’s 1-2 nights of play. You can’t really count the cost of talents enroute to obtaining knockdown against it since they have value themselves.