I don't believe you're supposed to use it to promote yourself or your product. But I've seen it used and I've used it myself when I want to bring someone's attention to something specific.

In general, no you shouldn't do that as it is considered spamming.  There are a few exceptions...like if you are answering someone's question and giving them a helpful link to answer their question.  But, if it's a link to your site, no don't do it. Most people will delete those links and you'll annoy them and lose followers.

Some folks get upset about it, and many will report you for it.

No idea.

Just do not do self-promo, Russ.

Nina nailed this one!

One should never promote themselves when leaving a comment for another person. 

I don't think an etiquette has been stated. I will hyperlink if I want to person to be notified about the comment. I do not hyperlink the author in the comment on his post. He would be notified twice.

If I have something relevant link, I ask permission and give permission to delete.

This used to be a major problem, but it has tailed off.  I feel that if you would not want someone to do that in your blog...you should not do it.  Also, therre is a courtesy of asking!

Russ Ravary links are there to 'supplement' your answer with another post/site, should you need to do so.

And yes, you are right - I have rarely used it.

I don't use that often, when I am answering a comment that someone has asked for a resource, then I add a hyperlink to make it easier.  In direct comments, I rarely do it.  I found it strange one time, when someone made a comment on a post and put a link to their posts.  Needless to say, I ignored it.

Use hyperlinks when the situation calls for more data.

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Be aware, those on AR are your peers. Sending them to a lead capture page whould be BOLD and comes with consequenses.

I agree with Debbie's comments.