While a staff master gets his defense bonus equal to his Con bonus for "wielding" his staff as an immediate interrupt. Not for using a spell through it.
An orb master gets to penalize one creature's save by your Wis mod to end an effect of one of your spells whether you cast that spell this turn or not. Just "wield" the orb in one hand and attack with a weapon in the other. It is a free action.
The only difference is that wand implement masters' ability directly relates to attacks while the other implements do not. So the wand wielder must only use his ability with his spells, while the other implement users do not have that restriction. None of the other implement users need to make sure that they are actually casting a spell to use their special implement ability, why put it on wand wielders?
Cheesy or not, it is valid. Wielding means having in hand for two of the three implement users, why should it mean using for the third?
Ciao
Dave