I switched to an open tank mainly because of ease of maintenance. It was a lot easier for me than having to take the glass top off evertime I need to reach into the tank for something. If using a glass top, it could definately get covered in grime or even water collecting on the bottom of the glass will diffuse the light and make it less efficient. If you do decide to go open top, just make sure your fish aren't jumpers
I only used glass lids. There is a certain amount of light loss through condensation forming on the underside of the glass lids but this is a pretty insignificant amount. Glass lids should be wiped down with a clean damp sponge every week & then dried before being placed back on the tank. You should have a sponge & towel dedicated to using on the aquarium.
Not using glass lids can result in large amounts of evaporation, especially in hot weather. The water that evaporates is pure, containing no salts or minerals. This will eventually lead to an increase in the hardness & mineral content of the water. With open topped aquaria the evaporated water should be replaced with RO woater to maintain a consistant GH, KH & mineral content.
I am going topless on my new tank. Reason....heat. I am with Funkman as well regarding cleaning. I may use glass in the winter and topless in the summer. I do a 50% water change on my tank each week so there will be a minimum accumulation of minerals (which I tend to lack in my water anyway)
Another reason to go without a top....it just looks good. LOL
I have both open tops and tops with covers. The ones with covers are mostly the fry tanks, breeding tanks to keep the temps more stable.
The others all have raised lighting and I like the way the topless looks. as was mentioned above there is a lot of evaporation but I top up often and do regular water changes. It is much easier for me to work in them spontaneously with out the tops ( I know I'm lazy LOL)