In it's current state the RadButton only emulates the looks of a MenuButton, but not the behaviour. In my point of view, such control should prove valuable to customers. More over, there is a growing need of base controls since we already have a at least three such implementations. A RadMenuButton should be the base (or it's base should be the base) for all control specific implementations of a menu button
Similar request: http://feedback.telerik.com/Project/108/Feedback/Details/149104-drop-down-button-mega-menu