I use both. The reason for both ambient air temp and a gradient is because of the frogs natural environment being warm. The air is very humid and hot. Providing a gradient allows them to choose whether they want to be warm or cool and not only this, but the cool side always hold a bit more moisture than the warm side so they can choose to be more hydrated without using the water source. Their digestive systems work by heat so if the frog want to be warm to digest a large meal it just ate it will go to the warm side and burrow. Their should always be a choice for the frog and if sacrificing the beauty of the tank provides a better home for your frog then that should be a easy decision to make.

When the UTH is on the side of the tank it should be just above the surface of the substrate so that it warms the air and not the substrate. The whole purpose of placing it on the side is so it doesn't warm the soil hardly at all. Anything that produces heat causes a drying affect and also will warm the substrate some because the ambient air is slightly warmer on that side.

There is absolutely nothing wrong with using these units combined with a light to create the correct climate for your frog.