If you leave the glass off the top you trap a lot less heat that can be transferred to your water. The pocket of air can act as an insulative layer and make it difficult for heat to escape. Saltcreep also can occur on the glass, blocking light, which is not best if you are keeping LR or corals.
A disadvantage for some, not in my experience, fish can jump out of the tank. Some will use eggcrate (lighting section of homedepot), but this can metal if you have intense lighting like MH.
Hope this helps.