Numbers always speak louder than words. If you can say that there are twice as many customers who are capable of finding and ordering products after a redesign, then your message carries much more weight than if you give a speech about the improved navigation system and streamlined checkout process. It is useful to track the evolution in the usability metrics over time. Keep measuring this, and you will drive up both the quality of individual designs and the organization's ability to deliver quality effectively and predictably.
Competitive measurements are often very convincing.
I know of one e-commerce project that was doing fairly poorly. The team decided to measure its own usability compared to a major competitor and found that it took customers 3-4 times as long to complete a purchase from their own site as compared to the competing site. With this huge difference in usability it was no wonder that customers preferred the other site. Usability measures are useful to support unpopular or controversial design decisions . For example, the draft home page for an e-commerce site had two navigation schies.