How important is it to leave the hyphen out?

If you have an affiliate site that you only hope to get SEO-related traffic from (instead of branding the site and building a name for it), then there's nothing wrong with a hyphenated domain name.

 

I've been working with online retail for a couple of years now and I believe there are so many other factors that would influence the customer before they would worry about if the domain had a hyphen in it or not... things like; a detailed terms and conditions policy, 100% returns policy, whether you can contact the company by phone, if they have a valid postal address, if they have a bricks and mortar store, and whether or not they store CC numbers on their system.