Scott - I'd say there are a couple of big motivations for removing glass tops (especially for reef tanks)...
The biggest is light. It may not seem like it, but even really clean glass will reflect an appreciable amount of light (and so will the water surface). Add some salt creep and dust onto the glass and you're looking at a really significant reduction. Most people don't want to pay to produce light that isn't getting into the tanks and most reef critters need the light...
Gas exchange is another. The reason evaporation goes down with tops is because the humid air gets trapped above the water surface and under the glass. This also means that less fresh air is able to get to the water surface and CO2 produced by the animals in the tank likewise cannot escape as easily. In many freshwater systems dissolved CO2 is a good thing because bad algae growth is less of a problem (less light) and many people are actually trying to grow algaes and plants (planted tanks). If you have a skimmer, plenty of flow, and/or an open sump this might be a non-issue... but it is one reason to go topless.
As far as your worry about heat, if your top has an open back I'd actually expect a decrease in temperature. By allowing the water to evaporate out of the tank and escape the hood, your water temp should go down not up. Usually glass tops kind of act like a greenhouse, which are intended to trap heat and humidity.
The one thing I'd watch with taking the tops off is to guard against jumpers. In most cases, a bit of gutter guard across the back will make sure none of the fish can get behind the tank.
