Cover reduce light?

[serratus]

I was just thinking, does having a glass cover on top of the aquarium reduce the light that goes into it since the condensation on the cover would refract and reflect some of the light away from the tank?

Is this why people buy stands for their lights so that they don't need to have a glass cover to sit on?

BTW, I'm just starting out so I don't even know if tons of condensation even forms on the cover but I do recall see condensation on the glass cover of aquariums in stores.

Without the cover, will the tank get noticeably dirtier faster due to dust landing into it? (i.e. you gotta clean your filter more frequently)