I have lost enough fish through jumping to always have my tanks covered. I have a eggcrate and/or glass tops on all my tanks.
Some interesting pieces of information:
* clean glass only absorbs about 10% of the light passing through it, so loss of light should not be a concern.
* eggcrate may concentrate light directly under a MH lamp, but as you move out from the centre it will decrease transmission far more than glass.
* if you have a skimmer and an open sump, gas exchange should not be a problem with a tank covered with glass tops.
* glass tops will reduce evaporation, but a fan over the sump quickly cures that - and works as a chiller, too.
* if you keep shallow water corals, UV is not a bad thing as long as you don't have more UV radiation than present in nature. Also note that UV-A and near UV radiation penetrate to 40m in the ocean.