Thermal control is critical during the saccharification process. The insulated lid helps to maintain the temperature during this process.
However, when sparging, the temperature is not critical to the scarification since it has already been completed. The typical sparge water temps around 168F are really to help "thin" the viscosity of the wort and to neutralize any continuing enzyme activity.
The important aspect of Fly Sparging is to maintain enough water on top of the "set" grain bed to hydraulically push the wort through the grain matrix. Too much or too little will negatively affect your grain bed. There is no one reasonable way for the home brewer to visually see the mash with the lid on.
The design follows the rules above:
1 - Temperature is [critical] during the Saccharification Process.
2 - Water Management is [critical] during the Sparge Process.
Hope that helps answer your questions on why we designed our Tun in the way we did.