Somehow I missed this post, but I think I'll respond anyway.
^_^

>I've been noticing more and more lately that people are 
>scanning their cels in the cel bags.  Is this really 
>necessary?  It certainly doesn't show the cel as well as if
>you scan it out of the bag.  And is there really a danger 
>(fading, etc) to the cel if you scan it once??  And if 
>there is, how does scanning in the bag help?

Scanning in the cel bag allows a couple of minor protective
advantages:

1) You don't handle the cel directly with your greasy little
   paws, and you're less likely to get drool on it. ^_^

2) Minor protection of the cel surface from things like 
   scratches from the scanner bed, too. 

3) In my case, because I use impossibly thick Mylar bags, 
   the scanner cover can't bend or crease the edges of the 
   cel that exceed the bed very easily. 

As for cels looking as they should, the only problem I've 
seen with cel bags are with the cheap and very flexible, 
easily wrinkled kind. My Mylars are invincible!! Color 
correcting depends on your scanner, software, OS, and 
monitor. 

I don't believe the scanner light is as dangerous as leaving
the cel in sunlight (or even ambient light) for long periods.
This also depends on the scanner - after all, the scanner
light is quite intense. The faster the scanner, the better.
I actually researched my scanner purchase with cels in mind 
(though it certainly doesn't hurt any other use).

The only way to test this is to get three junk cels from the 
same sequence with the same exposure history, scan one 
repeatedly while leaving the other in the sun or ambient 
lighting, and keeping the third as the standard...