Here is my opinion on it and I know for a fact that once I reply you will get at least one more response from another user.
However! :D So there are different types of blogs out there and WordPress sites - we know that not one is the same. There are however certain types of blogs that require more "discussion" than others. The bloggers who let their readers register are wanting people's input and comments for discussion. Now of course you can get comments without registered users but then you get lots of spam comments.
There are also the other type of blogs who let users post information as they find it. So for instance a blog that follows a popular actor could let it's fans register to follow and post sitings, pictures, etc as it happens.
Personally, I have that option turned off. My blog requires no discussion other than the occasional comment here and there - so I don't need to have registered users.
I hope this helped at least a little
Megan