"Stick to what you know"?

In management strategy discussions a familiar command is for firms to do what they have a core competency in. And it sounds sensible enough. Yet I've recently come across examples of firms that departed from "what they were good at" and became world famous as a result.

• Would we have Honda stop making cars and go back to just making motorcycles?
• Should Vestas get out of the wind turbine business and return to home appliances?
• And Nokia? Back to running a wood pulp mill?