Diamond plates have their uses in sharpening... They work well on soft (<57-58hrc) stainless Euro knives most normal people own, as they grind very quickly and linearly, cut very crisp bevels without the slight convexity you get on stones due to the slurry generation (Which makes the edges feel sharper than you would think, since there is little or no apex erosion.), are very low maintenance, and leave nice toothy edges for steels that cannot normally hold finer ones. They're also useful on pocket knives, especially those made from high-carbide content steels, as they'll cut practically anything and don't release any mud that can work itself into the joint. Lastly, since they stay flat, they work very well for the initial work on western woodworking tools (O1, A2, & D2 mono-steels, etc.); flattening the backs on chisels, and cutting in the primary bevel. I wouldn't recommend nickel-bonded diamond plates for sharpening or thinning hard, Japanese knives; they are very harsh on edges, and soft cladding wears out the plates very quickly.
The downside is, they do not have a satisfying feel, and the diamonds will slowly get worn down or pulled out of the nickel matrix; they last awhile, but not forever. DMT's used to be made better than they are today, but still are usable, and I particularly like the Fine 600-Mesh grade; it's kind of the gem of the range. Plates are also a little tougher to de-burr on since they are not conformable; your angle control, pressure, and stroke length has to be spot on at the end. A pasted strop is typically a great way to end a diamond plate progression, as this is a pretty fool-proof burr reduction tool. Making a large jump with a broken-in diamond plate from - say - a 600 mesh plate to a 1-3 micron diamond pasted strop to de-burr leaves a wickedly toothy edge!
When using diamond plates, it helps to add a drop of dish soap to your sharpening water to break the surface tension; when the plate begins to load up, scrub it with some scouring powder and a toothbrush to get it back to proper operating condition.
Hopefully this helps...
- Steampunk