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: 
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.
I would have to agree with balexandre. Although there are pros to using absolute links like less possibilities that a search engine messing something up etc, the advantages of using Relative links are greater for me because I won't have to change all the links if I copy whole portions of a site to another site, or even changing the domain name in the future.

So, for a complicated large website with over 8000 pages like mine, I use Relative links.

For smaller websites with less than 50 pages, Absolute links are slightly better. (if better at all)