If you have a cooler room temp, an overhead heat source is not a bad idea, but a UTH or belly heat source is still required. The reason some people do not need lamps is because there is a belly heat source in the low 80s on one side, and the low 90s on the other side. That is really all you need. My ambient usually hangs around 77, but the belly heat is in the right range.

Keep in mind, I DO use a lamp. That is because my apartment is very chilly. If the ambient room temp was around 75, I likely would not need the lamp because the heating pad and insulation on my tank would be enough to produce the correct heat gradient.

Overall reasoning - if you can produce the correct heat gradient without a lamp, go nuts.