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: 
Relative links allow you use less code to traverse your structure, and they allow you to move domains without having to do any extra work. You simply copy your directory structure to the new domain, and it works. 

As a lesser consideration, Absolute links don't make you worry about the position of your document relative to other documents -- which while not 'hard' to overcome, it adds an extra layer of thought that you have to be cognizant of.

Luckily the [Holy War](http://www.search-this.com/2008/01/16/2-minute-seo-tip-absolute-vs-relative-links/) isn't as big as other [programming holy wars](http://www.google.com/search?hl=en&q=Programmer+Holy+wars&btnG=Google+Search&aq=f&oq=).

For Email, your only real choice are absolute links.
I think when you're sending html based mail, you should always use absolute links, remember that the mail is going to be read in the email client program of the users, and the relative links will be useless unless you specify a [BASE](http://www.w3schools.com/TAGS/tag_base.asp) tag.