I have built two "antique" aircraft (see this link for a photo of one:
http://tinyurl.com/25jbkb ) and I can tell you from experience that
aircraft-grade plywood straight from the factory has a sort of sheen
to it -- a slick, hard, almost shiny surface that will not accept glue
very well unless that surface is dulled down by a light hand-sanding.

It is immediately apparent when you apply glue to a slick, unsanded
plywood surface that the glue is not "wetting" or penetrating the wood
very well, whereas the dull, softer, sanded plywood appears to "wet"
immediately and the glue is obviously penetrating it much deeper and
faster.  The plywood gets compressed by clamps or rollers during
lamination at the factory where it's made, and its outer surface must
be "loosened up" afterward to take glue properly.

In aircraft work you frequently make test joints that you deliberately
break when dry or cured to see how far the glue is penetrating, or to
let you know whether the "real" job is ready to come out of the clamps.
Plywood that is glued to other wood without sanding will break off
shallowly, perhaps only the first layer or less, whereas a joint made
with sanded plywood breaks deeply into the wood.

Of course, after the sanding and before the gluing the dust must be
entirely removed from the plywood surface or it will weaken the joint.
I use compressed air to blow it away.  I have also known builders to
moisten the plywood with wet rags to "open the pores" for the glue,
but of course if you do that you must then let the water evaporate
entirely before applying glue.  I never liked that method because you
are fooling around with the moisture content of the wood, which is
a factor in its strength.  I always felt you might be introducing and
then trapping extra moisture.

Wood (of the right kind) is a strong material for aircraft -- as shown
by the wooden-winged fighter planes of World War I, the Mosquito
reconnaissance airplanes and passenger gliders of World War II, and
the wooden wing spars used in all the early Piper Cubs -- but under
some conditions wood is not as durable over time as metal, mostly
due to excess moisture in some environments.

Wood can rot faster than metal can rust or aluminum corrode.  The famous
football coach, Knute Rockne, was killed in 1931 when a Fokker passenger
liner with wooden wings crashed in a storm in Kansas.  It was later
determined that those wooden wings had not been adequately protected
from moisture: water had collected inside over the years, and the wood
had deteriorated, weakened, and finally broken under the stress of
violent flight maneuvers as the pilot fought for control in the storm.

After the sensational Rockne crash, which was publicly blamed on wooden
construction, lightweight aluminum alloys replaced metal in all
airliners.

Tom Baker