Yes this does work best with those who already have some doubts, but it can cause doubts for those who aren't already doubting. And yes, its a matter of planting seeds. Its not a case of saying this reasoning "won't work" or it "definitely will". It can just be a powerful way of opening the door to their thinking, in my experience, and cause them to at least back off in being so smugly self-righteous about the org.
If the JW quibbles about 'new light' and the organisation of today having things more accurate than the bible students of 1919, then bring out how the Society has changed a lot since, say, the 1940's, for example, smoking was allowed back then and disfellowshipping was way more lenient and their was no blood policy and what were divorceable sex acts were different, etc. Then ask the JW "Was it Jehovah's organisation back in the 1940's"? Of course they will say yes. So all you have to do is ask them to apply the same perspective today and allow for the possibility that perhaps Jehovah is not very concerned about all these relatively minor errors so long as any group has got the main doctrines right? The JW may feel that only they have got the main doctrines right, but when they learn that there are still thousands of Bible Students who still believe JW's main doctrines, they often pause and think.....
If they want to hark on about Rutherford's reasons for branding the fallen away Bible Students as "evil slaves" then that leaves prime opportunity for you to analyse Rutherford's reasons and ask how could it be that tens of thousands of Bible Students, who the Society says were all anointed in 1919, suddenly turn from being the only approved Christians on planet earth into "evil slaves" in 15 short years or so. A reasonable minded JW (which most of my relatives and friends have been, thank goodness) can see this can seem rather unreasonable. Point out that if Russell had lived for a few more years and Rutherford had taken control, then Russell would undoubtedly have been one who would have left along with all the other tens of thousands of Bible Students? If the JW doesn't agree with this hypothetical proposition, then pull out some quotes of Russells was opposed to highly centralised control and the entire concept of "organisation", so he would have been opposed to Rutherford's new strict control and increasing ruthlessness. If you produce the exact quotes of Russell on this to make it clear that Russell would have been disturbed at the new direction Rutherford was taking.
If all else fails then pull out this trump card: If C T Russell was alive today he would be summarily disfellowshipped for apostasy! This point can really hit home with a JW who is already experiencing some doubts. At least in my experience.
Just a suggestion anyway. In my experience it is a waste of time trying to argue with JW's over their doctrines as a way of causing doubts...it is better to focus on the organisation and help them get a more balanced, objective perspective of it. This is one approach to do that.