You dont really take the chain off the saw every time you sharpen it do you?
It's a PIA if you have more than one chain per saw and you want them all touched up / sharpened before you head out in the field. You don't want to mount/unmount each one. A clamp like that would also let you leave your saw in the shed, truck, etc., and just bring the chains into the shop, basement, (kitchen if you are single), etc.
I have some old bars that I used to use with an Oregon, bar mounted file guide (similar to the Granberg file guide), but the chains move around a lot unless they are clamped or under tension.
I would be interested in a clamp like the one shown by super3. Anybody know who sells them? Will keep an eye out for the old style saw sharpening clamps like drill.dady referenced at garage sales.
Philbert