I agree. I am far more intimidated by INTJs than by INTPs. I rather enjoy debating INTPs and to some extent ENTPs because their style of debate tends to be very polite and conversational. I'm not saying INTJs are mean or less cordial or that NTPs can't be brutal, but INTJs tend not to waste time or words with too many platitudes; they also don't seem to second guess themselves...if they know they are right (or at least they think they know they are right) they go in for the kill, whereas an INTP will step back and consider their opponent's point of view and second guess himself/herself. I think such second guessing gives the (false) impression that the INTP is perhaps unsure of himself/herself or not wholly convinced of their own argument's validity, thus blood is in the water, which the opponent smells.
I might add that INFJs can also be intimidating, although perhaps for different reasons I don't want to get into at the moment.
That's not to say INTPs can't be excellent debaters.
I found this video linked at Celebtypes and it's amusing to watch the way two INTPs (yes, I agree with their assessment of Ben Stein as an INTP) debate one another. Each of them basically tries to trip the other one up by asking what could almost be considered "gotcha" questions to show fallacies in their opponent's logic. That style of repeatedly questioning one's opponent can be highly effective. I wouldn't dare put myself on the same level as Stein or Dawkins, but I've noticed that this is something I tend to quite often and naturally in "real world" situations, something I did long before I typed INTP, and it can be quite frustrating to other people. It's a surefire way to make my father red in the face. I don't usually do this with that goal in mind, so when people react with anger or frustration, it always confounds me. It was largely because of this that I was the bane of many teachers' existence during my early years.
Anyway, sorry if I went too off topic with this post.