A glass cover on your DT will reduce your gas exchange and in turn, cause a drop in PH. Evaporation is normal for all tanks, but in SW
it is especially important to keep an eye on because as the water evaporates, the salt is left behind. Too much evaporation and the salinity level in the tank will go up. Many people (myself included) top off with fresh water daily to replace the water lost through evaporation. As for the reduction in light, yes, it can reduce the amount of light entering the tank. Glass gets dirty and can develop a film on it, so unless you clean it everyday, you'll get some light loss.
Edit: Many people use DIY covers made of plastic porous material like lighting grid or plastic mesh. This is especially useful if you want to keep adequate ventilation and keep any jumpers from carpet surfing.