The biggest issue for me about laying out a cal course on site is when it has to be done. I like to arrive at the course site as early as possible, usually just around first morning light, so traffic is as light as possible. If I have to spend the first 60-90 minutes finding a good location for a cal course, measuring a cal course, and then calibrating my bike, by the time I get to my second measurement the traffic has increased significantly. Regardless of what people say, it takes at least 60min to do all of that, more if things don't go perfectly.
The only accuracy advantage of doing the pre-cal onsite is avoiding the change in cal constant in the time it takes to drive to the course. If you are arriving at the race course at first morning light, the temperature usually changes very little in the two hours or so before that.