Using string to measure camber

Several people told me they measure camber with a string running from the top down. Now if I put a weight at the end of the string, I'm sure the string will be perfectly perpendicular to the floor (assuming my floor is flat).

Is this a valid method? In my opinion, as long as the string doesn't swing and the weight did not touch anything other than the string, I don't see why this won't work.

If this measurement is valid, why I don't see any racing team doing it this way? Is it not a good technique?

This is what we call a plumb bob, and it gets used quite extensively, although not for camber measurements (that I know of anyway). It's true that it will always be a totally vertical line. The reason you don't see race teams using a plumb bob is that it would be too slow and cumbersome to use. You have to have a convenient place to hang the string from and have to wait for it to settle and stop swinging each time. If you touch it even slightly with the ruler when trying to take measurements you will have to wait for it again.

So in principle you could, but it isn't very practical.