It’s one thing to use a piece of clipart to spice up a banner or sign, but quite another thing to incorporate clipart into a brand mark or logo. In fact, it’s a very bad idea.
There are almost always copyright issues associated with clipart. Many clipart collections have stringent usage restrictions and may even specifically forbid use in a logo design. For example, iStockphoto prohibits the use of any royalty-free images purchased from their site for logo designs. This is stated very clearly in the iStockphoto license: “You may not … use any of the Content as part of a trade-mark, design-mark, trade-name, business name, service mark, or logo.”
Most clipart collections, whether ‘royalty free’ or not, retain rights to their work. They may license an image to you, that is, let you borrow it, for a limited range of uses, but you cannot own it.

If a company uses a clipart image as part of a logo, not only does that mean that the company does not have complete ownership rights to their own logo, but they may be exposing themselves to a possible infringement claim. Besides, using artwork that everyone else can use—does this not defeat the purpose of branding?
Obviously, images we use in creating a logo or mark should be original artwork, or artwork to which we own all rights.