My little 2 gallon bowl (with a waterfall type filter) would lose about a cup of water a day, I can't imagine how much a 40 gallon breeder would lose with all that surface area. On top of that, you have to wonder where all that water is going. If it doesn't condense on the surfaces of your house, it's going to make your air conditioner work harder in the summer. Also, your tank heater(s) are going to work harder to keep it at temp.
But my main reason was that it gave me the ability to fill the water up to about 3/4 of an inch from the top and not worry about fish jumping out or water just plain splashing out and hitting me or the wall or landing on the carpet. I was worried that over the years there would be staining on the wall or carpet from the constant drops landing in the same spot, especially from the filters splashing.
If for no other reason, look at all the water that's on the bottom of the lid...that would all have evaporated out of the tank instead of dripped back in.
Having said that, I'm going to set up a QT tank and likely won't put a lid on that unless I can find a really cheap one.