If your wondering why wales isn't full Dirtyfighting(lethality) its due to a little imbalance. The middle tree dot for gunslingers (shock charge) affects wounding shots. Where as the Sniper middle tree dot (interrogation probe) does NOT affect cull.
REALLY? Well that is convenient for Gunslingers then.. But on the other hand if you look at it, regardless of whether or not Cull procs off our 3rd DoT, ~it should not be
that much of an issue since there is no way of putting out more DPS at this point other than
1. using less rifle shots (which can be attained by)
2. having enough energy all the time from DoT crits (higher than normal crit rating)
Cull is still used in the same part of the rotation and will not change in order or priority even if our Interrogation Probe magically starts to proc it the way it does for Gunslingers (if what you said is true).