It seems that it would be nearly impossible to have an idea of how many potential readers you don't get because they don't want to register. I know that from time to time I will point my wife to a post or an article but she can't view them and has never bothered to register. She works as a development PM so her depth of knowledge about sql does need to be very deep, but she does need some level of understanding.

Of course you have reasons to enforce it just as people have reservations about registering. I am usually able to figure out a way around it but it would be nice.