as the salt creep develops on the lid it can decrease the intensity so it just requires more cleaning. but the biggest advantage of not using them, is it helps to keep your tank oxegenated. as stated above the biggest disadvantage would be security, jumping fish or eels may get out .
I do not have my lids on. i get more evaporation since i do not use them but i prefer them off. I have a canapy so that protects my fire fish on all but one side from jumping out. the last side has quite a few peices of equipment on it so helps to deter jumping. Have not lost a fish yet to jumping out. but its always a risk. Im gonna have to rig something up on the back side when i get an idea or others here have used eggcrate light diffusers to help prevent jumping.