Whether you're developing an application within its own brand or for a client, implementing creative elements of the interface with CSS creates the potential for resale value while minimizing the labor required to rebrand. Depending on agreements made (or not made in some cases) you, your client, or both parties will own the application.  The ability to quickly rebrand increases its value imensely. This enables the developer to ask a higher price and the owner to attain better ROI. 

While it's advisable that a developer should always use CSS, developers know well the temptation to render an image tag instead of using a link containing hidden text then setting a background with the stylesheet.  Taking the extra time to ensure each and every element of the design is customizable through CSS can make the difference between a $5k application and a $50k price tag.

Next time you get a request for a custom web-based application, consider the ablity to rebrand that application and think: "How much more will rebrandablity benefit the owner?" Ultimately, this will let you fairly sell the development and production labor for a higher price.