I think everyone should have one and not necessarily for white balance. Use it to meter on for perfect exposure. Just set the card where the subject is going to be and spot meter on it. Using manual exposure of course. If you do this correct then your exposure should be perfect. Well, at least a really good starting point.
When I owned a photo lab I would always shoot the grey card at the beginning of a roll of important film. Then process it and then I would set up a channel on my printer based on the grey card. I would get the most beautiful photos on those rolls. They were always perfect.