The advantage of having blog comments is that they are the easiest way for people to, well, make a comment on an entry. The main alternative is email, but blog comments are significantly easier than email in this modern age of spam and other problems, where not only is it difficult to find the blog author's email address (because if it was easy to find the spammers would find it), but it's difficult to know if you can trust them with your email address.

(While writing an entry of your own can be easy, my feeling is that it's not an effective way of commenting because there's no good automatic way of bringing it to the attention of the original author. In theory trackback would solve this problem, but in practice it has drowned in spam.)

When you make things more difficult, fewer people care to go through the effort, especially first time people, and as a result you'll get fewer comments. Mind you, sometimes this is a desirable state of affairs; there are drawbacks to comments, especially a lot of comments, and that's ignoring the entire issue of comment spam.

(Note that someone who 'comments' regularly has less problems; they will know your email address or other way of getting your attention (or not care), they will have already decided to trust you with their email address, and so on.)

It is my guess that you will not necessarily get a better class of comments by making commenting harder; you may even get a worse one overall. The problem is that you're not selecting for people who have something good to say, you're selecting for people who care enough, including people who have a pet cause that they will only be too happy to tell you about. (On the other hand, it is generally easier to filter such people out of email and other ways of attracting your attention than it is to filter out their blog comments, and this effect may not kick in until you are fairly well known.)

(Obligatory attribution darnit: this entry was inspired by this, which started me thinking about the general issue.)