I have said before that no panel is made to be perfectly straight. The large corves in that roof panel are an area that the LONG block may help with. The idea of those long blocks is that they make the curves more uniform and gradual. Since they are in fact flexable, you would need about 10 hands to hold pressure on the whole block while moving it. You could, however, on a large curved panel, place one hand in the area of each end, and cause the block to bend to match the general curve of the panel. That way, you would be removing some of the slightest "steps", that you can get from sanding many different areas of those curves, and try to blend them together.
That being said, I am still not going to run out and buy one. The "steps" that I mentioned, when the work is done by someone that knows what they are doing, would not be noticeable by eye. If you made an exact set of templates, you would likely be able to see a difference across the panel. Then again, you cannot get it perfect anyway, without a set of templates, and working yourself to death. Not worth all of that to me.