I think dividers can be quite useful on their own, but can be more useful when used in conjenction with other tools - rather in the same way that a marking knife is more useful used in conjunction with, say, a try-square or sliding bevel.
If dividers are used in conjunction with a sector (as Alf pointed out earlier in the thread),or with a good rule, they can do even more than if used on their own. They can make their own rule, of course - set dividers to any arbitrary length, step off along a clean, straight stick, return the marks all round the stick with square and knife/pencil, and voila - one homemade ruler!
What Jacob says about setting dividers by 'trial and error' is quick, and plenty accurate enough for most woodworking tasks. It isn't accurate enough for engineering tasks, but that need not concern the wood-only person at all. Wood doesn't work like metal, and doesn't behave like metal, so the techniques are different.
If anybody does need to set dividers accurately for some metalworking or similar purpose, they can be set within a couple of thousands of an inch by carefully placing the (sharp!) points in the engraved lines of a good quality steel rule. Adjust until the points drop neatly into the middle of each engraved line (the screw adjustable ones, used with a light touch, are useful here). You can either use a magnifier, or 'feel' the points in the line - hard to describe, but easy with a bit of practice and a light hand. Transfer the mark with delicacy and feeling - dividers are prone to flex if used with a heavy hand. This technique is not, of course, necessary for wood marking - the stuff will change dimension more than two thou if it comes on to rain the same afternoon!
