This way, prior to the end of the boil, I know if I'm way off my target gravity. This means I can supplement with DME at this point if necessary. Also, since the wort is still boiling, I can add the samples right back into the kettle so I don't lose any wort to the various diagnoses.
Good idea adding the DME. When mine was low I just continued on, and ended up with a Scottish Ale at "session brew" strength. It still tasted pretty good, and folks seemed to like it.

Scott