Dina has a good blog post titled Conversational Blogging. In it she says "I find I am so let down when I read a post without comments".
There has been a lot of discussion about what make a blog. Is it the personal voice, or is it that readers can add comments? Or is it about linking to other posts?
The first thing to realise is that it doesn't really matter whether your blog is really a blog or not. The only thing that really matters is that you have some interesent in posting and your readers have some interest in reading it. Whether you call it a blog or a frequently updated website is really irrelevant.
That said, how do I identify a site as a blog rather than a website? For me, I agree with this post by Amit that the main thing is the distinctive voice of the writer. But all this is just to name the page as a blog or website, which like I stated above is really quite irrelevant. If the page is compelling, it doesn't really matter whether you call it a blog or not.
Now, coming back to the comments part. Some blogs don't really need comments. This is especially true of link blogs which just provide links or reproduce other posts with maybe a small commentary. However, blogs with essay type posts greatly benefit from having comments open.
Very often I've encountered posts that I would like to have commented on but the comments are off and I can't really get down to crafting out an email. What's more, I can't see what everyone else thinks about it either. The result is that there is no real conversation. The flip side is comment spam/trolls. Its irritating for sure. Blogger doesn't have any good method of preventing comment spam, but if you're hosting your own blog there are some solutions to it.
So comments are definately needed if you are into the whole "blogging is conversation" idea. If you are not into it, then you can get by without comments. I definately think that the power of blogging is the conversations that it generates, and therefore have my comments on, but if you don't believe so then thats fine too.
Here is a small example to show what I mean. Read Amit's original post, and then read this cross-posted version with comments enabled (read the comments too). Which one is more enriching? My vote goes for the version with comments.