I've just come back to FW from the SW side and SW tanks/reefs are open-top affairs. So the goal there is to keep lights as close as possible yet out of the reach of salt spray.
The one PITA I had forgotten about was cleaning glass tops. If you maintain Africans, the hard water may very well permanently etch the glass and seriously impede light penetration, but it's less of a concern there. I keep potted plants in my angelfish tanks and periodically clean the glass tops to keep algae from forming on the underside, and dust from accumulating on the top. So sure it can impede light penetration. How much just depends on how dirty they are.
I've been told they do block a good bit of light but my experience has been they do not, so unless your light is marginal you shouldn't have any problem. I have a 150 with no lids and it evaporates about 2 gallons per week, this 150 used to be covered and I've seen no difference without the glass and my 2.5 and 10 gallon tanks are covered to keep critters in and slow the evaporation.
As 150EH points out, the trade-off is in being able to control evaporation. If they are kept clean, the loss of light penetration (there has to be some), is minimal. And if you keep enough tanks, the evaporation becomes a whole 'nother set of issues to deal with.