As Richard said, brake fluid draws moisture from the air. That moisture will corrode (rust) your steel brake parts, including the steel tubes. It takes a long time, but it's not a good thing. I just replaced a brake line on my old Toyota because it sprang a leak. I bought it well used and the brake fluid probably had never been replaced until I got it. Too late!
BMW motorcycles have a maintenance schedule for replacing brake fluid. It's realatively cheap, and easy to replace. I do it. You should too.
I have never changed power steering fluid, but I think I should. The reason I haven't is because it's difficult to do. The turkey baster method will only change a small amount, so you really never change it. But... It's better than doing nothing. I'll probably get around to it this summer.