bob is correct. brake fluid should be routinely changed. period. a simple answer to how fluid gets contaminated is that the system is not 100% sealed, and things like moisture(let alone dirt) can be introduced to the system by simply openin the cap.
a more indepth explanation would first involve knowledge of all the definitions of contaminate, not just dirt/moisture in the fluid:
con·tam·i·nate Listen to the pronunciation of contaminate
Middle English, from Latin contaminatus, past participle of contaminare; akin to Latin contingere to have contact with — more at contingent
1 a: to soil, stain, corrupt, or infect by contact or association b: to make inferior or impure by admixture 2: to make unfit for use by the introduction of unwholesome or undesirable elements
so, referencing the bolded definition, if you have old, boiled, or worn out fluid in the calipers mixed with the "good" fluid, the system is CONTAMINATED!!
each pad change is an oppurtunity to remove the portion of fluid held in the calipers. this is the majority of the fluid in the system and is a good way to routinely cycle the fluid and keep it fresh. crackin the bleeder when you compress the pistons leaves very little chance of allowing air into the system when performed with a clear hose attached to the bleeder. i still do a quick gravity and/or pressure bleed after a pad change in this manner in most cases for peace of mind.
if you think this method is inefficient and will slow you down to the point that it may cost you money in the field, keep in mind how much money a comeback may cost you in the event of a part failure after a "simple" pad change where contaminated fluid was forced backwards thru the system. not sayin it will happen everytime, or any single time in your career, but it is possible.
i have done pads both ways for a long time. after noting the condition of the brake fluid in most vehicles comin thru the shop, i choose to start crackin the bleeders and at least ensure that amount of fluid is fresh everytime.