The forced-login isn't to improve product quality, it's to make more money on ads. StackOverflow/Exchange works great without forced login to view content.
Stack overflow works better when people provide answers and feedback on prior answers; persistent login greatly increases the chance that someone will leave a reply, which in turn helps future users.
You never wondered why there is very few fake account on stackoverflow as compared to ad based social network?
Registering non participating members is only a trick to inflate your user base when communicating. What really matter is active users, and they will register don't worry.