This is one of those rock and hard-place type choices. You have to make a choice about whether you would prefer to deal with spam or potentially reduce real comments.
The rock: I started my blog back in November. I don't require a sign-in. I currently receive about 10 spam comments for every 1 real comment. On the plus side, the spam filter catches almost all of them.
The hard-place: There are several blogs that I have visited where I would have left a real comment, but when asked to provide a log-in I gave up. I don't imagine I'm the only one. I don't know how you would measure the impact of this.