Depending on the field, the calculus and differential equations may not be necessary. I haven't used calculus since undergraduate school, but I do use logarithmic functions quite extensively.
I'll agree about the obsessive part. At the beginning of grad school, one of the professors told our group that if we aren't absolutely passionate about the subject, we were just wasting everyone's time. He was so right about that...
Well,I haven’t used them much since I got out of school. But I had to learn them to get out.
I was never particularly passionate about my dissertation matter. It was a means to an end. But I do know how to grind :)