Base reflectivity versus composite reflectivity

Thanks to Rod Machado's article in May's _AOPA Pilot_, which was a
recap of another aviation safety enthusiast's lecture, I just learned
that there may be a big discrepancy in a storm's intensity when
comparing its base reflectivity (what is actually falling out of the
cloud) to its composite reflectivity (what is going on inside the
cloud).

The probability is greater, this article implied, that the composite
reflectivity will demonstrate more intense activity inside the cloud
than the base reflectivity depicts. This, in turn, implies that a
pilot of a small aircraft deciding to penetrate precipitation based on
base reflectivity may, in fact, be penetrating an area of stronger
activity.

Furthermore, the article mentioned that most (if not all?) current
downlinked/uplinked weather products only display base reflectivity due
to the attractiveness of the overlay on the display (less blocky in
appearance). This might lull some pilots into believing that
convective activity is non-existent or weaker than it really is.

Of course, those of you who fly aircraft equipped with active radar
knew this all along, but those of us low-time GA pilots being sold the
current uplinked/downlinked radar solutions may not have been aware of
the differences.

First, a rather basic question: Why is it that all of the moisture
that appears in a composite reflectivity map is not all falling? Is
this due to the strength of the updrafts within the cell? It seems to
me that moisture that appears that heavy on radar would be heavy enough
to all fall out of the cloud.

Secondly, when ATC reports specific levels of precipitation, am I to
assume that they are giving us base reflectivity? What about when FSS
gives us precipitation levels en route? Base reflectivity only?
Should we be specifically asking for composite reflectivity?

Looking forward to learning more about this topic.

--
Peter