Double opt-in is recommended for two reasons:
1. As a marketer, you are sure that the newsletter subscriber is a real person and not a robot.
2. For some websites, not securing the sign up process with double opt-in could lead to bots spamming the community.
It's really up to you to use double opt-in or not. For some clients we recommend it, for some we don't.
On this website, we don't use double opt-in to register an account, as we have a league of other strategies that keep the bots out and spam out.