If you or your clients are targeting small to medium businesses, a recent survey entitled "Optimizing Email Newsletters for Small/Medium Businesses" has some useful information for you.
According to the study of over 300 executives, email newsletters rank highly as sources of information, beating out websites and blogs, and matching print media for importance. A weekly or monthly newsletter was the preferred frequency, and 'how to' and product information the top content areas requested.
This is some more valuable information you can use to explain the benefits of email marketing to your clients. Although the study specifically focused on small to medium (less than 500 person) businesses, it would be safe to extrapolate that out to most businesses and consumers.
We'd be interested to know how often you or your clients send your newsletter - have you had the best results with monthly news, or weekly? Or something completely different?