As all of you aware no doubt aware, once a Psyker has mastered a discipline they receive a -5 to all Thresholds, this along with the Focused discipline Talent can make some supposedly major powers alot easier to manifest to the point where they become a no brainer choice over similar minor powers, making the choice of choosing the minor powers a wasted choice.
Some examples for you,
Divine Shot, Threshold is 15, Mastered takes that down to a 10, when combined with Focused Discipline, it will require only a 8 to manifest. This is equal to Unnatural Aim, now baring some specific situations, which is better OVERALL?
Another example, Seal Wounds over Healer, Not only is Seal Wounds better in term of reaching the Threshold, by having an effectively lower Threshold, it gains overbleed as well, not to mention it being only a half action as well as healing more damage.
This only makes it more likely that players wanting more "efficent" characters choose minor powers that have no corresponding Major power making the character in question spring these major powers from nowhere as they have had no experience with the lesser version.
Now i full understand that in some cases the major power should be flat out better, nor am i saying that once assigned to a discipline that they should have a -5 Threshold adjustment for mastering a Discipline. But something that i think would help the situation is having the them benefit form Focused Discipline, as a minor thing, being associated with a Discipline will have with the in character justification of some powers also.