Probably not a great idea, you're only getting the top fluid out, the real damage is occurring in the calipers, where that fluid is baking everyday from the heat of the pads transferring through the caliper pistons (that's where you'll find the nasty stuff).

But don't feel bad, if you got your brake fluid "flushed" at a dealer, there's a good chance that's all you're getting anyway. The good old turkey baster treatment...just suck it out of the reservoir, clean off the bleeder valves with some brake clean (so it looks like you did something) and you're done in two minutes, paid an hour.

I once ran into a guy that would skim a half quart of oil every oil change he performed. The dealer began "counting" the amount of oil we pumped into cars, we'd have to go to the parts counter, tell them how many quarts we needed, and that's all we'd get. People were stealing oil out of the pumps so the dealership started doing this - well this guy still got his - he'd just pump a half quart into milk cartons he had in his bottom drawer, then use them when he did his own oil changes. Every car...half quart low...usually if you're getting your oil changed at the dealer you are not the type to go checking your oil, and if you did? Well he just made a simple mistake. Long story short...change your own fluids if you can.