And no top can...
1) add a lot of warm humid air to the room (increases AC costs, increases chance of mildew)
2) lead to more evaporation and lots of replacement water
3) make the warm water accessible to insects, children and pets
4) remove a barrier that might protect the tank from insecticides and other sprays
5) help the tank collect unwanted dust and smoke particles more easily
What's the upside?