Are drop bars suitable for winter commuting?
Drop bars in of themselves are fine for winter riding. However, there are some peripheral issues involved. Your average flat bars will offer a bit more control in slippery conditions because flat bars are much, much wider than drop bars. A bike equipped with drop bars may not be the best bike for winter riding. But properly configured and sized drops can be excellent for winter riding. 

Handlebar Width: Drop bars tend to be narrower than flat bars. You can mitigate this by getting wider drop bars; the ones meant for touring are great for winter riding. 

Keep in mind that handlebar sizing isn't an exact science, but you generally want them to be as wide as your shoulders. A lot of road bikes won't have anything approaching this kind of width. (Also, there aren't that many wide, stocky roadies.) 

Tires: Road bikes--which is where you usually see drop bars--often won't allow for the fitting of appropriate tires for riding on snow and ice. Not the fault of the drop bars!