I definitely agree that the best customers are the ones that leave and tell you why. I also think most, if not all, of the "I quit HN and here's why" posts are from precisely the type of people that makes HN worth reading, and the complaints they have are generally real problems that HN would be better off if it could solve.
With that said, I think there's a difference between quitting HN and unsubscribing from say, a online service. When you unsubscribe from a service there's a clear demarcation. Quitting HN is usually a decision of the form "from now on I won't go to HN", a decision which is very easily reversed. Because it is so easy to reverse one's quitting, it doesn't carry as much weight.
On the other hand, let's say pg stays off HN for a year. Eventually people will ask why he quit HN. If he writes an essay about how he came to quit HN and why (past tense) as a response to that question, it's more likely that he has quit for real, as well as being true reasons for why he actually did quit.
Another example: This is my last cigarette, because smoking is bad for my health vs. I quit smoking 10 years ago when I had kids and realized I wanted to be there for them when they got older. It's true smoking is bad for your health, but that might not be the true reason people actually do quit.