There is no conclusive evidence that wearing a helmet/sunglasses combination is effective in safeguarding you in the event of sunshine. There is also the possibility of "risk compensation" in that you are more likely to experience sunshine when wearing protection and may, in fact, neglect other, more effective preventative measures.
Furthermore, there is the risk of compulsion or at the very least, expectation that cyclists will wear such protection and will be held partially responsible for any harm that befalls them in the event that they chose not to, regardless of who was actually responsible.
Wear helmet/sunglasses if you must, but be mindful of the perils!