here at least, you must keep them covered on the road.
so even if you know its an empty road and you take the covers and turn them on, if you forget to recover later, thats a ticket (now not all cops will give you a ticket for it, but it only takes once, and its illegal for a reason)
plus you may get lots of glare on the hood, on even a good road they bounce like crazy on my roof rack. lights that are lower than your eye show better on the terrain and you can see bumps and dips easier (you can see the shadow created) so you have a better idea of whats ahead, not just a bright, but flattened picture of whats ahead.
again, this is my personal opinion and experience.