It's easy to forget the hyphens when typing a name. Many users are used to typing things like rendc.org but not global-guide-line.com. They'll probably leave out the hyphens and end up at your competitor's site.

When people recommend your site to their friends verbally, having hyphens in your domain name leads to more potential errors than when the name does not contain hyphens. For example, how do you think your visitors will refer to your site if it is named "global-guide-line.com"? They might say, "I visited Acme Book and Videos dot com yesterday. It was fabulous." Their friends, remembering that comment later, might type into their browsers "rendc.org".

Oops.

It's a pain in the neck to type. Enough said.