Hi, Fred..... I'll try to give some insight into the reasoning...

Even though the water is emulsified in the fluid, it can still do it's dirty
work..... years of witnessing rusty wheel cylinders and calliper bores (and
the odd perforated brake line) attest to that. AFAIK, DOT5 isn't hygroscopic
and one of it's major downfalls is that it doesn't bleed easily.... once
fine air bubbles become entrained in the fluid the resist coalescing and
coming out (these bubbles are formed by simply pouring the product).

The minimum spec boiling point for dry DOT3 is 401 deg F. Most brand name
fluids exceed this spec but not by a great deal, IMHO. The heat load imposed
during a good downhill coast or stop and go driving can raise the fluid
temperature significantly, reducing the available buffer zone for panic
manoeuvres. As little as 6% moisture content can reduce the boiling point of
the fluid to about 260 F. A dragging brake can really through a wrench into
the works.

Racers get real familiar with brake fade quite often but brake fade isn't
just a concern for racers. It can and does happen on the street and it is a
condition that we have a means to avoid or at least reduce - changing fluid.
Most of us will find a need to service our brakes at roughly the same time
that a fluid exchange would be a wise service - making opening the bleeder
when retracting calipers an easy way to start the process.

As far as storing fluid is concerned..... a refractometer can be used to
measure water content but low water concentrations can turn the task into
guesswork. When it comes to such an important system as the brakes, I can't
see saving a buck or two as being worth the gamble....... not to me, not to
my customers and not to my family. The foil cap under the brake fluid lid is
hermetically sealed.... the same can't be said of the little cardboard or
plastic disc taking it's place. Let's not forget that temperature variations
of the container may have it "breathing" quite regularly.

The question isn't whether we can or can't use fluid from an open
container - it is a question of whether we SHOULD or not. I'm in a position
where I cannot and will not take that chance. The liability concerns alone
are enough to weaken the knees.

It remains a question of safety and fresh fluid from a sealed container is
about the safest thing we have when it comes to brake fluid...

The official Ford recommendation on changing brake fluid is that it's
not needed other then what is done during normal pad/shoe/brake
service.  Ford based their recommendation on studies that showed
moisture content in brake systems rises to about 3% fairly quickly and
then stabilizes.  Changing the fluid just provides new fluid to fairly
quickly absorb it's 3%.  

I agree with your thoughts on the claim that once opened a can of
brake fluid should be tossed.  IMHO it is the mechanix equivalent of
an old wives tale.  Oft told with no basis in reality.  If you open
the can, use what you need, and recap it there's nothing to worry
about.

--
Elbridge Gerry, of Massachusetts:

"What, sir, is the use of militia?  It is to prevent the
establishment of a standing army, the bane of liberty.  . .
Whenever Government means to invade the rights and liberties of
the people, they always attempt to destroy the militia, in order
to raise a standing army upon its ruins."  -- Debate, U.S.  House
of Representatives, August 17, 1789