Do you have a policy about publishing comments on your blog posts? I won’t publish comments which are:

Nasty;
Have no real value, such as “great post” – I’d like to know why it’s great, and what would make it even greater; otherwise it just looks like spam, or an attempt to draw your attention to yourself;
Anonymous – I’m happy to publish comments anonymously at the person’s request, but I’d still like to know who made them; that seems to me to be both courteous and sensible, because I may wish to follow up or clarify a point. I also think that if someone has a point to make, especially a negative or controversial one, there is something slightly suspect about not wanting me to know who they are;
Obviously spam, which could be in the name, the web address or in the comment itself (like the one I received recently which read “Great post. This is an important issue, which is why these handbags are such excellent value.” Eh?
Offensive in any way.

I have a written policy that covers the last one of these, and an unwritten policy to cover the others. I also have comment moderation enabled.

Advantages of a comments policy

It helps to prevent the site being spammed – or, at least, the spam actually appearing on the site. I get tons of spam, which is a back-handed compliment I suppose.
It helps to maintain the intellectual integrity of the site, by insisting on useful comments as far as possible.
It can help to maintain intellectual honesty if you insist on commenters posting with their real names (I don’t, because some people would be worried about their employer seeing their comments – as long as I know who the person is).
It helps to prevent bullying via nasty comments posted anonymously.
It maintains commercial integrity: why should companies use your space without paying for it? You wouldn’t let them erect an advertising hoarding in your front garden, would you?
If written, it sets out what you regard as acceptable, and marks you out as a professional.

Disadvantages

There is a perception that if you undertake moderation, you can be prosecuted if something inadvisable is published, ie if something falls through the net. I am not a legal expert, but I’m pretty sure this isn’t true (disclaimer: don’t take my word for it because like I said, I’m not a legal expert!). But besides, surely the likelihood of that happening is greater if you don’t moderate? In any case, do you really want your site to gain a reputation for allowing spam or legally actionable comments?
Time.

On balance, I’d say the pros outweigh the cons, but I’d be interested in hearing your views on this.