Thought I’d toss in my two cents on this one…
For low end scanners (and to me low end is 40 pages per minute or slower), it’s likely that any jpeg compression will be occuring in the host PC, whether it’s done by the application or the driver. That’s a matter of cost, and because jpeg is very well defined it doesn’t matter from an image quality point where the compression occurs.
For faster scanners the bottleneck is I/O, and in that case jpeg compressed images coming over the wire may be the only way to maintain the speed of the scanner….