Dissenting opinion...
Glass tops are handy. They're pretty much the most fool-proof way of preventing jumpers, but also they can do a lot to prevent much evaporation. Lots of people will tout the benefits of evaporative cooling and they're absolutely right about that... but there are times and situations where being able to limit the amount of top off water needed is a good thing. If you don't have an auto-topoff device and you do much travelling, having a glass lid makes vacations less of a trauma on the tank. If you're worried about your water bill and all the waste water from using your RO unit, using lids means you'll use a lot less. And if your source water for top offs isn't the best quality to begin with (for whatever reason), needing to use less of it over time is a plus.
If you keep cats, lids can be a neccessity. If you have small children with a tendency to drop things into a tank, lids can be a neccessity. Lids also stop airborne pollutants from landing in the water... all the dust, pollen, pet hair, glitter from your 2nd grader's art project gets stopped.
Lowered pH is largely a myth. Can it happen when using a lid? Sure - assuming you aren't using a protein skimmer or even an air stone somewhere in the set up. Passive gas exchange at the surface is reduced when using a lid, but injecting air and the surface turbulance resulting from it pretty much negate that as an issue.
It will increase your tank temp compared to running topless, definitely. But if you're keeping any truly escape-prone fish, need to keep something out of the tank, or need to limit tank evaporation, lids have their uses.
