I mean, after all we have a GPS and a compass.  And maps (if not
charts) and probably a guide or two (eg., quimby's).

We're going to do our first segment of the great loop next spring
(Lake Michigan to Mobile/New Orleans).  Since we'll be on fairly
major rivers with a significant amount of traffic, how important is
charting software?

What's frustratinig is that there is some fairly decent shareware
(http://www.sping.com/) that uses commercial maps, and Fugawi
Marine ENC which uses free ENC maps (in addition to
comercial maps), but costs $200.  Capn wants $795 for the
Great loop charts alone!
I realize that we're not supposed to balk at spending money,
but I spent a lot on my Garmin ique already.

http://home.comcast.net/~satyrical/

I fully agree with Al that they are certainly not a necessity. But they
could be considered a pretty handy "option".
It's not all that hard to lose track of where you are on a river. I don't
mean relative to the channel or between the banks but which mile marker or
landmark you just passed or are approaching.
When Gwen and I came up the Mobile, Tenn-Tom and Tenn. from Pensacola to
Guntersville we tried to keep a little marker, in our case a small
locomotive player's piece from a Monopoly game, positioned on the chart
relative to our progress. We moved it every few minutes as we moved along.
There were times however that we would get distracted by something and first
thing we knew we didn't know where we were. Not a big deal, just grab the
binocs and catch the next mile marker. No big deal that is unless we heard a
tow boat announce he was headed toward us at Jones Bluff and we don't know
whether we're going to meet him in two minutes in the middle of the next
bend or ten minutes away along a nice straight stretch cause we didn't know
where we were.
We've found that using the electronic charts relieves us of the need to
continually move our little loco along the paper chart in order to keep up
with where we are.
Bill