Question: When creating links for an an html based email, how critical is it to use absolute links? Also, What are the benefits in using relative links in general?

Answer: 
Email? I don't think there's an option other than absolute links. This will guarantee you that no matter who and how will read your email, your links will point to the place you need.
I like to use a constant for my top level domain, and then add on other parts to the link. This is just in case I change my top most domain name, but would like to keep the content/link intact.

For example:

COMPLETE\_SITE\_NAME = "<http://www.mysite.com/>";
link = COMPLETE\_SITE\_NAME . "pagetoContent.html";

Slightly slower, but a little bit more flexible.