Its a waste if you've attracted the prospects attention, gained their interest, and built their desire with all the things that it can do to improve their lives or their companies and you don't ask them to do something specific, such as buy your product, come to your store, send back a form so your sales rep can call, etc. I recently got an offer of one months free membership of a local sports club. A free trial is usually a very effective offer. The format ties in very closely to the objective of the campaign. Most direct mail asks for a response and the format of the mailing should make this easy to do.
Debate continues over the use of the envelope.
My experience has been that the most effective campaigns communicate the key benefit in a thought provoking manner. In this age of glossy photographs, it comes as a surprise to most people who are new to direct mail that the letter remains the most important part. The letter does the selling job. A well written letter should grab the reader from the first line and then take them effortlessly all the way to the end. The Brochure does the same job as the letter, gain interest, communicate the benefits, explain how it works, and close the sale.