Compare being in the field with a kydex vs cordura sheath, they do retain dirt and moisture MUCH more than kydex/securex/boltaron. While hiking up to your stand you get your knife and sheath all muddy (you may not have this issue in the city of course). You can just rinse the mud off of the kydex and shake it dry whereas the cordura not so much.
Sorry I was thinking about all my leather sheaths. Kydex of course is a different thing altogether and in fact I have Kydex on all my "Real User" fixed blades.
I tend to wear a fixed blade when fishing and sometimes I'll gut clean before going home. No matter how much in-site rinsing one does on the knife afterwards,
the smell stays and in the case of leather or cordura it stays there. With kydex its a simple matter of washing out the inside.
No question kydex is a better actual use material. My point was intend to say that Cordura is no better or worse than Leather.