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My impression is that when you open the bottle and dump some out, that missing liquid is replaced by the current air - which contains moisture. The moisture in that air migrates into the remaining fluid. Again - please send me your open unfinished bottles because the amount of moisture is generally trivial. The idea that the bottle of liquid is sucking water out of the air around it (in the timespan of a year or two) is borderline preposterous. Open the bottle every three days and I'm sure it will pick up much more moisture than if it sits sealed for a season or two. If you are in a 100% humidity environment it's going to pick up more moisture than if you are in 15% humidity.
The second thing someone mentioned is contamination. Use the bottle with greasy/dirty hands and leave the cap off and you increase the chance of getting junk in there. Smoke a cigarette over the bottle and drop an ash in there. A fly thinks it's water and lands in the bottle. A lot of things can contaminate the bottle. You use the bottle instead of a container to bleed the brakes and the line gets dirt in there, or backfeeds old fluid in.
As a general rule - it's OK to keep the remainder and reuse the stuff. You won't have massive brake failure as a result. Consider the # of times the bottle was opened, the environment in which is was opened, and the age of the fluid. After a pile of openings (if you get that far before emptying it) or a good number of years - pickup a fresh bottle.
The study cited above is for a canister (of a SPECIFIC composition that is not necessarily the same as what brake fluid is kept in) that is immersed in 100% water. I recommend against storing your brake fluid in the pool.
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