I was waiting on his response and I was too lazy to blow the pic up to see the seating surface, but now that you did and I have, notice the circled area. Does it appear to be running out? Basically the distance between the seating surface face and the head appears to be getting smaller. Like it is wearing off center. (Another reason to suspect the guides are worn) The face appears to be concave, which is typical wear and the seat is a mirror image of this. This is the reason why just replacing valves is not the best method for longevity. Might be wearing off center, but that's not really so unusual early on near the end of service life. The coating, which, for those who aren't aware of it, is normally a lot less than .001 thick, is simply going to wear through in one single spot, which will then extend the rest of the way around the valve face fairly quickly. The uneven nature of the wear often leads to some degree of "scrubbing" as the valve closes, and that puts a lateral (relative to the stem axis) twist on the stem at the valve head. That can lead to he separation of the valve head from the stem, which is almost always a disaster. Not good for the guides, either, even if the stem doesn't break. The guides in YZF's and other "bucket tappet" engines last a very, very long time absent any heat or lubrication problems, because there is normally little or no lateral thrust force born by the the guide, unlike engines that use rocker arms.