Product marks, service marks: which are more "sacred"?

Reagan Cook writes: "Tony: I’ve been reading your reviews of logos trying to understand some of the basics behind this art and I keep coming back to a central question.  Is the importance of a logo, or the sanctity of the logo, more critical to product-based businesses than service-based businesses?  I ask this because so many case studies or examples seem to come from product companies." 

First, Reagan, let's be be clear that we're talking about category brands  (product or service logos), not institutional brands (corporate logos).

In product categories, the image of the product itself ... or a product design attribute like Caterpillar yellow ... can be more important identity factors than the logo.  But in service categories ... finance, health, broadcasting, whatever ... often the only thing you can actually see is the logo; in your mental picture of the brand, the logo tends to take on more importance. Think Merrill Lynch without a bull. So in general I'd say logos are more important in service categories, where you don't have product design to help in visual differentiation. (The exception is retailing, where you do have architecture to work with; think Starbucks.)

Corporate brands, of course, are different. Unilever is a "products" company but the Unilever logo represents the corporate management function, and is essentially a service brand. In fact, where a corporate brand is also one of the corporation's product category brands, it's often a good idea to use visibly different product and corporate logos (for example, Ford / Ford Motor Company).