It would be decent for an attack build if it were Total Speed and not cast. Attack builds just can’t give up their exclusive skill slot just to get the ADCTH given the alternatives. Or just give up the Cast Speed altogether and give it a defensive stat or crit damage. To me the skill is trying to be a little bit to a lot of builds (most cast builds can’t take advantage of the ADCTH or flat damage and the weapon builds can’t use the cast speed) and just isn’t good enough for any build to take whereas the 12% OA by MoD is good for everyone.
I always felt that Master of Death, since it’s mostly for pet builds, should have had % DA instead of OA.