Allow me to throw in some of my bias. I have not read the entire thread, but I did read Kevin's post above. I think it's a very objective perspective on the subject.
I told Shred Ready they ought to make those things for their helmets but they blew me off even though I clearly I know more about helmet making than anyone else in the world.
Haha, that sounds like something I would do. They might listen when they sponser you and you risk your sponsership blogging about it or something.
Maybe do that after you've given them a couple more emails/voicemails/phone-rants, given them a chance to respond, then boom, "I'm going public with this!" Okay, not saying anyone should actually go to those lengths. There are good options out there as it is.
I like my WRSI. I have the plastic full-face attachment (the old one) but found that I basically don't need it, although it would protect against abrasions I've yet to encounter.
Although I didn't come away from my first classes with much kayaking ability, Alex (of Confluence) did touch on the subject of helmets. One point he argued, was that he has to tuck in when he flips to protect his pretty face, because he's paid to be pretty. Obviously, kind of joking there and it did illicit some chuckling, but he made a good point. If you develop the proper reflexes for a flip, there's not much need for the full-face. I've known people to come away from a swim or being flipped in general, with a bruised face, or a missing tooth. Whether reflexes and proper tuck could have prevented it, I don't really know, but I do know that up to this point, my face, glasses and all, has always come away unscathed; still want to eventually get LASIK, but I digress.
With that said, I fully endorse people wearing full-face helmets on the river. There's a slight increased risk of entanglement, in theory, but I'm yet to hear of that occurring. Never had a problem hearing people speak through them. We know we have to talk loud amongst rapids anyway (if we talk at all). Mostly, I only run into trouble with the ones who wear the ear-plugs; just gotta remember to yell louder or stop to reconsider if what I have to say is even worthwhile.
I use my kayaking helmet for the slopes too, but up until recently, I never wore a helmet anyway, so I can hardly complain. Would've loved my WRSI when I got that snowboarding concussion at the age of 11. All I learned from that at the time, was not to let myself get distracted. The idea of wearing a helmet never entered my mind. If you really lost control and were pushing the limit in the wrong spot and landed on your head, you'd want a motorcycle helmet, or equivalent; maybe a motocross neck-brace as well (still very out of fashion, at the time of this writing; but they do make them).
My perfect helmet right now, would be the standard WRSI with the latest Shred Ready retention system. I might have to frankstein that helmet together, at some point. Although I might want to change out the possibly-questionable mono-filament lines with some strong bow-string or stainless steel cord. I do have a friend whose helmet rode up his forehead too much in the wrong spot; causing a big swollen bump on his forehead, after greeting a rock with it. I think the Shred Ready retention system really helps mitigate that risk. Maybe it's just me, but after that story, I've been constantly tightening and retightening my WRSI when I have the time. It still bobbles up and down a bit if I force it.
As for my motorcyclist-side (strictly-speaking, currently I'm a scooterist), I value light weight in a helmet a lot. Hasn't really been a consideration with kayaking so much, but laws of physics don't change. Having less mass helping your head meet up with the asphalt rendezvous point is always a plus. I value my brains at about $500, so that's how much money I'm willing to spend on a motorcycle helmet, give or take; to make it lighter and hopefully, safer.
Again though, I haven't spared such consideration for the multi-impact world, but the theory should carry over the same, even if it doesn't actually make a big a enough difference to generally matter. I suppose it might, if you're going down those super-steep slides though; like the one where Paul broke his ankle. Yes, Pizzle, word gets around.