I think it depends on the person and the conditions.
I personally get along really well with the one other INTP I know. He is a really good friend and we clicked right away.
The reason for this instant connection may be that we both live in a small community and go to two different small schools where there is no one else like us or who has the same personality type.
We both are largely misunderstood by our families and classmates so we understand each other. The community we live in has very traditional values so I think we get along well because we don't have to "chameleon" or hide our eccentricities with each other like we do with everyone else. We both understand the pressures from our traditional community (he, a seventeen year old guy, likes knitting and I, a seventeen year old girl, likes robotics and hates makeup; we both receive crap from people for not conforming to expectations).
I think we also get along well because we are both surrounded by people who are feelers and it is nice to say something without someone getting upset. I think we both deal with having both parents possessing the dominant function of introverted feeling, which clashes with our dominant introverted thinking.
Also, our community is more focused on sports than on science and nerd stuff so we can talk about our interests.
So, to answer the question, I would say that INTPs are good friends for each other because it allows us a safe haven to be ourselves and to be understood.