It might be worthwhile to think about the things that could get a knife handle wet during a knife fight.
Yes, it could be raining, particularly if you live near me. So, plain old water.
You could slash a guy in the groin area. That could get you blood thinned by urine. Other slashes could produce only blood. Blood is fairly slippery stuff. Just play around with some raw steak for awhile, or buy a couple living chickens or rabbits and butcher them yourself.
The right slash could produce vomit which would project onto your knife hand. I haven't handled any more than obviously necessary, but think puke could be fairly slick.
Of course, the potential that diarhhea (sp?) could happen to you or your opponent and that could get onto your knife hand, exists. Up around the face, there is saliva and snot. And puke. And blood. And eyeball fluid.
For me, self defense knives should have the maximum traction available in the grip. Losing your knife in the middle of a knife fight probably means you die. Pure and simple.