Or any sort of controversial subject. There are two types of people whom it is a waste of time to have these sorts of conversations with. Those whom you agree completely with - you spend half a day saying "yeah, that's right" to one another and get nothing done. The other is those whom you completely disagree with on everything related to the subject of the conversation, as you will spend hours explaining that the sky is blue, and they will hear "The sky is purple", and they'll do the same thing to you.
The best type of person to have these conversations with is someone whom you disagree with on some issues, but agree with on others. Even if neither of you changes your mind, you'll have both given the other some food for though.