variable spacing means that you can choose the width of your workpiece, say a drawer, and divide the pin spacing for a perfect layout. with a fixed jig you are forced to work within a very limited number of widths for good spacing.
a cab maker who pumps out drawers as fast as possible from the same plans week after week might like the simplicity of a fixed jig. a nut like myself enjoys pumping out drawers about 1/2 the time, and the other 1/2 i design and make custom furniture and my d4r leaves all the options open for me. i also use fixed jigs when the appearance is not priority, or when i'm making sliding dovetails or box joints. sometimes i ad a wide tail or even fade the pins into tails and back to pins over a workpiece giving the piece a very custom look. as opposed to the s t a m p e d o u t look a fixed jig offers.
i have also made the mistake of "nicking" my jig with a bit, (not waiting for the bit to stop before removing router) with a fixed jig this mistake would be very costly. with the d4r, i just ordered an extra slider. the shipping cost me more than the part!
then theres the manuals. i think god wrote them himself. mortals just don't think that clearly. i have a small fortune in tools, the majority of which i have not read the manual for. but i love to read my leigh books,