In most systems, there is very little ciruculation/mixing of brake fluid. This is important because brake fluid slowly absorbs copper from the brake lines. As the copper accumulates and in the presence of water and heat, the fluid then begins to cause corrosion/rust. The oxidation material (including rust) is not flushed back to the master cylinder during normal operation. This results in pitting of the cylinder walls and errosion of the seals. A significant reason for flushing/bleeding the brakes is to remove the fluid before copper accumaltes to a harmful level (there are test kits availble just to keep tabs on the copper).
Depending on the climate and driving style, I suggest that bleeding the lines is more important than replacing the reservoir fluid.
I will grant that fluid replacement at the master cylinder is better than doing nothing (some line fluid does get back to the master cylinder), but if your goal is to really make a difference, then bleed the lines at least every other year. I have found a yearly gravity bleed to be fairly painless.