I'm working on a site which allows users to sign up for a service. They choose a username/password, enter their personal details, etc and proceed into the site, their details being stored for future visits.

Currently, we don't ask for confirmation of the user's email address like most websites do. 

The main function of the site is to process payment information for the service being offered but the client will likely want to expand the functionality of the site in the near future.

So my question is, should I be implementing an email confirmation system for new users and what are the risks to the site if I don't?