I've thought about -adding- a folding knife to my pfd as a backup and for other uses, but I would -not- take away the external knife. A few of my thoughts, purely based on empirical experiences of myself and others:
Roy's is the classic, but a recent story that's a reminder of why an easily accessible knife is good: good boater was doing a class IV run and got randomly snagged by a rope buried just under the surface while boating downstream. It's unlucky, but not that far fetched on popular runs. I gather that he managed to grab a tree branch with one hand to keep himself stable. With the other hand he grabbed his knife, but couldn't get a good angle on the rope to cut it.
So, he decided to put the knife back in the sheath to switch hands. He said he had the NRS Pilot knife, which stays very secure in the sheath, but apparently is hard to put back in the sheath with one hand (disclaimer: I've never never used one) and was having a lot of trouble. Anyway, eventually the force of his boat or something freed the rope and his boat and he was released without having to cut it, but obviously the situation could have gone downhill fast.
I've been using the blunt tip Bear Claw with serrated edge and haven't found a good enough reason to convince me to switch yet. I seem to lose one just about every year, which sucks, but I just consider it the price of admission. I've gotten good at finding deals at hunting stores. I'm still reluctant to tie the knife to a leash - I know some people claim w/ the proper length leash it's pretty safe, but I've had some violent hole rides and I don't like the idea of a knife flopping around attached to me.
With the Bear Claw, my casual observation is that taking it in and out of the sheath a lot wears down the tightness of the connection so I try to minimize the amount of times I take it out. Running waterfalls also seems to be a great way to lose your knife, as is climbing around on cliffs or on top of logs. I've also learned that going into salt water and not washing off your knife afterwards is a great way to ruin it. By exercising a few precautions you can keep your knife around a bit longer.