If Koth of the Hammer's plus one ability ends at end of turn, doesn't it make it useless?
I think you've misunderstood the ruling:


  If Koth's first ability animates a Mountain that came under your control that turn, it will have "summoning sickness" and be unable to attack. It will also be unable to be tapped to activate an ability with the {T} symbol in its cost, such as the Mountain's mana ability or the ability granted to it by Koth's emblem.


If the mountain has been under your control since before the beginning of this turn, it does not have "summoning sickness". It's only unable to tap and attack if it's a mountain that you played this turn.