I don't mind them too much. You are right about blocking the light though. I reduce my floaters to a patch the size of a half dollar, and in a week it has almost taken over the tank again. I am fine with it because a lot of my tanks fauna can be found in the roots of the duckweed. Baby shrimps and snails hang from the roots and little critters swim beneath it like a small upside down forest. Really fun to watch!