It's interesting to me how far apart opinions are. For me, as a buyer, it's unlikely I'd buy something used without running it through Carfax. I think to many peoples points, there are inaccuracies or missing information, but something is better than nothing. It's also a good check against what a seller is telling me.
As a seller, I would still be hesitant to email it, not really for any good reason, just a gut feeling. I can't stand emails asking what my rock bottom price is, come see the car in person, if I'm going to waste my time the buyer should too.
IMO a big part of the question has to do with where in the lifecycle a car is. I've typically sold stuff at end of life, and pretty much make it know that it either has a couple of issues, or it's old and may have issues in the future (if asked). I love reading those ads on craigslist that say how solid the card is, just needs a new tranny or how this saturn with 240K will never leave you stranded.