I pretty much only break out the pressure washer when my car is FILTHY. That doesn't happen a lot. If I was detailing as a job, I would foam every car, but for me, that's kind of a luxury step. Probably like most on here, my car gets washed a lot...it's generally still "clean" when I wash it, and it always has a nice coat of wax. In that situation, foaming the car (IMO) is unnecessary, as the pre-wash rinse, combined with a good wash mitt and soap will float the dirt off nicely.
Also, my water pressure is pretty bad, so the extra spray and power is nice when I use the pressure washer.