I believe that originally it was thought that the controller should give horizontal avoiding action and then let TCAS deal with the vertical. But recently that has been changed (at least in the UK as far as I understand) so that the controller passes traffic but gives no instruction and lets the pilots sort out whatever they think is best. Presumably the thinking on this is two fold.. firstly giving a turn whilst climbing is going to reduce your climb rate and secondly having two instructions at once to follow could increase the chance of a mistake and worse happening? You end up with the Uberlingen problem of two voices screaming imperative instructions at you. You might only hear one of them.