I get the impression Vista Business and Ultimate will let you share scanners just like printers, but I haven't tried it yet. I took my scanner apart, and for one reason or another it only worked once after I put it back together (I actually think it's a software problem though). Anyway, I can definitely see the point of a networked scanner...
1. Wait for the scanner... Scanning a full image in high resolution takes a long ass time. It you were able to use the scanner software you could do a test scan, find the area where you want detail so you only take the time to scan that.
2. Wait for the network... High resolution images are quite large, transferring them over a network could get annoying
3. No switching programs... Sometimes you don't want the scanned image for long, File > Import > TWAIN from within your graphics program is a lot more convenient than browsing to a folder and opening an image.
4. Obviously 2 computers that are next to eachother, with equal access to a shared scanner makes a lot of sense. I personally like to get my documents as close to square before scanning them which takes a lot of adjusting. If you had to do a full scan before every adjustment it would take many times longer.
I'm with Carol
here, if all you need is "a scanner" than go for a cheap one. I used to have a Visioneer 9020, cost me something like $100 IIRC. By the time I bought my current scanner I could have bought it for like $20. My current scanner was $250 retail (Epson Perfection 4490) I didn't pay that much though. I think I'd be hard pressed to tell the difference in scan quality. But, if you need a large bed, or better negative scanning, you gotta get what you gotta get.
I wonder if a USB KVM switch (~$30) would work?Or something like this thing