Apparently that's not sufficient, or not the best approach. The Y.M. explains that the lessons of Torah will not penetrate an unreceptive heart or mind. Unadulterated Torah truth is not always the best answer. One must "know" what to say, how to respond, which means sometimes offering different answers than what one might say in the beis medrash.
I don't work in kiruv, but from what I have read it seems that kiruv seminars are more akin to theatrical performance than university debating matches. To critique arguments presented by kiruv lecturers as not philosophically precise or not l'kol hadeyos of halacha or hashkafa is to miss the point. Rigorous philosophical of halachic truth and a "response to the apikores" are two completely different arenas.
I'm not so concerned about a kiruv professional "pulling one over" on an apikores and getting him/her as a result to be shomer mitzvos. The road to return will be filled with many changed opinions and impressions, and ideas that grabbed the ba'al teshuvah initially will hopefully become more refined and sophisticated as their religious thinking progresses. What is more disconcerting is bnei Torah who accept answers that may be appropriate in the context of "da mah she'tashiv" as being the absolute truth, not subject to question, dispute, or alternative viewpoints, without realizing that greater amkus and critical thinking is necessary on their part.