I understand the properties of brake fluid and the "possible" consequences.but I really don't think it is worth the money to pay some one else to do it or the time for you to do it unless you happen to be working in the area (ie replacing brake lines) or you take the car to the track often.
I have changed brake fluid in cars before that were over 10 years old with 100k + miles with no perceptable change in braking performance. Also I don't really think any modern cars brake system will "rust out" if you don't change your fluid every few years. It has a higher chance of rusting from the outside imo.
So if you are changing the brake lines or calipers, by all means swap your fluid too, otherwise worry about something else.