Some have the opinion, that especially a large booms deform a lot when under stress. The deformation in fact shortens the boom (measured inside the boom) in strong winds - and that's in circumstances when you need it least! A few years ago some windsurfers experimented with connecting the back parts of the boom tubes with a string (through the sail!), but the trend quickly disappeared again - probably because the shortening of the boom under stress wasn't that pronounced.  You can see a little study of the shortening effect here. In practice the boom shortening effect is (in my opinion) negligible, and the "problem" has obviously diminished further with the new generations of very stiff carbon booms (for instance MS Carbon Race). Important or not important, the "flaps-up" strings shall to a certain degree counteract the boom shortening effect.