Personally, a few weeks ago I would have agreed with you Andy but now I am not so sure.
Kydex as a sheath material is rot proof, will not easily become a hoist to germs, will be more resitant to damage and stretching ect ect - it doesnt have the asthetics of leather but in the long run I think it might just pip leather at the post.
As for wooden scales going with leather - the Runt in its sheath looks ok to me but it you really dont like it why not use the kydex as a liner and stitch a leather sheath over it!! ?? That what I did with my first Mora classic.