I've always wondered about that as well. I wonder if that's why straight pipes seem to dominate the estate pipe sites; people buy both but don't smoke the straight pipes as much, making them easier to refurbish and resell. Just a thought....
Like others here, I prefer slightly to maybe 1/2 bent; they just feel better. Of course I smoke when out walking, so that might make a difference; if I was sitting and smoking, I might prefer something else.