The most useful threads on CF for me are conversations with people who share the majority of my beliefs, but who differ from me in some way or who have some personal experience that I can learn from.
When I have conversations with Christians who hugely disagree with me, these differences are often so significant that we just can't get started into a fruitful discussion.
And, of course, if I'm talking with someone who exactly agrees with me, the reinforcement may be nice, but I probably won't learn much from the discussion. I learn best if there's some kind of different experience or perspective the other person can bring.
(I'll note that genuine interfaith discussions are often a positive experience. When I'm talking with, say, a Muslim or a Wiccan, and we acknowledge up front that we follow different faiths, I can often learn from the other person's perspective. Maybe the difference here is that the Muslim or Wiccan isn't trying to tell me what Christians should believe or do.)