1. You would only have Boulder crush as your main stun ability.
2.Dark flare traps don't stack with other dark flare traps so a main class gunslinger will always have a higher level dark trap than you thus making your traps do 0 damage.
You might be saying that only way to kill those geared out tanks/bards is to use a dark flare trap, BUT here is the problem, how the heck are you going to keep them on it if you don't have a stun? (assuming you use the -10% def mastery). No decent person is going to just walk up to your dark flare trap and sit on it while it ticks on them for all their HP.
3.If you choose the gunslinger mastery for -10% def you are giving up your 3sec stun which has a 1.5sec cooldown. and also think of it this way, It takes about ~17/18k def to cap at lv 69.
4. Gunslinger skills have too long of a cooldown if you're a subclass thats not a SPD build and they don't really do that much damage as a subclass anyways. Sub classes are meant to be there for utility purposes, not for damage.