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YES, you should change the fluid periodically. I personally change mine once a year. It is recomended for a street car that it be changed every 12,000 to 15,000 miles.
Brake fluid is Hydroscopic thus like you said, it absorbs moisture. The Moisture lowers the boiling point of the fluid. Brake fluid becomes compressable once it contains moisture and or boils which causes a spongy peddle for starters. Whats even worse is that once it becomes compressable, the same peddle effort gives less line pressure and thus less clamping pressure (i.e. less brakes)... This obviously is a safty issue.
DOT 3 fluid typically has a dry boiling point of 370*f, at just 2% moisture content by volume, the boiling point is lowered to about 320*f, and at 3% will bring it down to about 290*f so at just 3% your down 80*f which is significant.
DOT 4 is a bit better because of two aspects, for starters it starts out with a higher dry boiling point (typically about 450*f) and it absorbs moisture at a slower rate which is good. The problem with DOT 4 though is it looses boiling point faster at the same percentage of DOT 3. At 3% moisture content DOT 4 typically below 300*f but the higher dry point and slower absorbsion rate makes if a superior fluid to DOT 3
Any1 who says that changing the fluid is a bad idea should not be working on brake systems and or probably cars for that matter. I have never once had a leak form because of changing the fluid even on cars that i know have never had it changed or not had it changed in many years. Also these "mechanics" should sit down with brake system engineers and try to argue with them. Or even better, contact the NHTSA (National Highway Traffic Safty Administration) and see how many accidents and deaths are out there that have been proven to be cause by sudden brake failure due to boiled brake fluid.
Rust from brake fluid having to much moisture in it is what causes the seals to go out 90% of the time. Under the pressures that the systems typically operate at, sludge build up that would supposedly seal small slits and what not keeps the all sealed as the pressure would blow it out. There is a reason why the o-ring are Square and cut and not round that seal the pistons and I'll bet that anyone who says changing the fluid is a bad thing probably can not tell you the correct two main reason why they are square cut.
For the safty of yourself and other that are either in the car with you and others that are on the same road as you at any given time, change the fluid and bleed the brakes well.
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