They do this on certain cameras, look up "Dual ISO". Effectively they make every other line more sensitive, increasing dynamic range when they merge the lines t

They do this on certain cameras, look up "Dual ISO". Effectively they make every other line more sensitive, increasing dynamic range when they merge the lines together. It's not a perfect solution, moire and other artifacts increase, and you lose resolution. The trade-offs are often worth it, especially for video.