There are several reasons why double opt in is necessary, but a few key reasons I believe make it mandatory. The double opt in confirms that the person who entered their email address actually wants to subscribe to the list. If this extra confirmation is not present (single opt-in) then any person could enter any email address to sign-up any other person.
Imagine if you upset someone and they decided to get revenge on you by entering your email address into hundreds of pornographic email newsletters. You would be forced to abandon that email account (unless you really really like porn) if these lists were single opt-in.
From the email list owner's perspective it's important that only people who genuinely want to be on your list are signed up. If you use a single-opt on method and, like the example I gave above, someone entered hundreds of email addresses of people who did not want to subscribe to your list, you would end up with a very poor quality list. Those "spam" subscribers wouldn't respond to what you were offering since they are not your target market.