Recall the infamous Cartier-Bresson thread on the Flickr DeleteMe group?
Of course we want to avoid accidental camera shake, missed focus and blur caused by faulty equipment. But there are at least two types of blur that any photographer will want to use routinely.
The primary use of creative blur is to make the subject stand out, to separate from the surrounding clutter. Controlling the depth of field in a portrait using wide apertures allows only the subject to be in sharp focus, creating visual contrast between the foreground and background.This type of blur is called "boke". Some photographers enjoy this type of blur to the point of obsession!
There are several techniques aimed at showing movement through blur in an otherwise static shot. Effectively this is intentional camera shake.You can move horizontally or vertically to blur linearly, zoom in or out as you take the shot for and impression of forward or backward movement,or pan along with a moving object to blur the background while keeping the subject sharp.
If you have a flash you can use a "shutter drag" technique where part of the exposure is blurred with camera movement while the flash is fired so the subject is in sharp focus.
I guess there has to be some rules for those of us still cutting our teeth in this business. Accidental blur is not always a good thing, but used sparingly intentional blur can add greatly to an otherwise ho-hum composition.