I guess I've always thought it would be easier to cut the trap line if it was stuck on my harness hook. The lower part of my traps going to the dogbones are line, not wire, as I suspect most are. For the range of motion needed to get out of most harnesses (over or under the vest) I've also thought through cutting the webbing on the harness to release the buckle. I keep my safety knife mounted to my vest where I can get it one-handed with either hand.
Of course, probably the real danger of the harness is that the hook can get inadvertantly caught on anything when you capsize, not necessarily being hooked in to the traps.
My brush with this was a pitchpole in which a sheet wrapped my ankle. I easily unwrapped it but it was enough to make me think about it!
In the recent Sportscar magazine (SCCA monthly rag), there's an article that talks about being mentally prepared for what happens and what to do during and after an accident. Once things begin to go wrong, your actions may make a difference between life and death. Thinking through these situations and what you might do in response to them before hand may help you make the right decisions with less panicking if doo-doo does happen. Perhaps this a good lesson for sailors too.
I too like the idea of a combined vest and harness. Except that if you do need to get out of your harness, you'd lose your flotation as well. It would need a way to release the buckle/hook without compromising the function of the vest.