1) Gandalf is often reluctant to give away too much info at once, plus he may
not be completely certain what has happened at Isengard since his last
visit. But he also i think privately enjoys this, and relishes surprising
the others.
2) I think it started as good-natured rivalry between the two, and turned into
a bonding experience. I agree with some of the others in that rereading
that part i found it a tad disturbing, even though they were discussing evil,
vile Orcs. The "body count" aspect, which i thought cool when i first
read it, now seems a tad disquieting. I also agree that it seems to come off
better in Jackson's movie, perhaps as humor relieving the grim reality of their
predicament.
3) I think Tolkien uses this to show how mankind generally fails to properly
appreciate the beauty and utility of nature, be it caves or trees or streams,
etc. It also shows that Dwarves are more complex than we first thought,
and also helps in the bonding between Gimli and
Legolas.
