Damn, what a nice, cordial discussion about hydrometers. Use one if you want, but don't dismiss those who don't as being wrong. I use one, because I respect the brewer who is so in tune with his (her) beer, and so confident in his (her) skills that he (she) does not need one. I am not that awesome yet.
it's not about being in tune with your beer, really.
especially for doing all-grain, it's the only way to measure efficiency (or with a refractometer, which is the same gig.)
it's also the ONLY way to measure attenuation, alcohol percentage, etc.
now, if someone leaves their beer for a month and goes straight to keg or bottle, they are probably fine. if someone tastes their sample and think that it's good enough to keg, that's fine too.
in other words, you can brew beer just fine w/o an hydrometer and noone truly "needs" one, but there are certain things you may wish to do that are impossible w/o one and it remains as the only true way to see where your beer is at