Dutret,

The point is that you need the right geometry for your racing position, not for more casual positions on the bike -- one rides a track frame on a track to race it and not much else. You can be racing in a 40 km points race for about 40 minutes and never have your heart rate below 160 bpm or your speed less than 30 mph. You have to be in the drops the whole time or be disqualified. If you are bent over a shade too far, you lose lung capacity and will quickly fade. And while a kilometer time trial may be a very short event lasting barely more than a minute, if you are just slightly constricted in your breathing, you may only lose by 0.1 or 0.2 seconds but that is often the margin between 1st and 5th.

If you aren't racing, then skip all this. But if someone is buying a keirin frame with racing in mind, they need to consider this. And it's a bit crazy to demand keirin geometries but then to size the frame such that one gives up the whole point of the geometry, and the reason why it's been optimized to where it is.