Hey - I'm the author of this piece. Thanks for voting it up to the front page. I love finding cool science in ordinary/everyday places, and am curious to hear people's responses to the piece. Cheers.
Picture a leaf in the Amazon, do you picture a big, waxy leaf? I do. Rainforest = more rain = more wax.
Alternatively, heavy rain and wind can flip leaves upside down, blow plants over, and splash dirt and water up onto the underside of the leaves, so maybe it is an advantage for the whole leaf to shed water.
I agree with your thoughts that this could be a consequence of coating the top surface in the waxy needles. However, I think it's actually somewhat unusual for a leaf to be superhydrophobic on the top and bottom, so I was wondering if there was some specific advantage to this leaf. Also, the needles on the under-side seemed far more dense than those on top, which was interesting.
One, a superhydrophobic surface reduces the surface area of the water that lands on the plant, allowing less of it to evaporate and more of it to fall to the ground.
Two, it may help prevent plant diseases by allowing less water to attach to surfaces that are prone to becoming diseased.
1. More surface area = better ability to capture carbon dioxide from the air.
2. More surface area = heatsink effect = reduced heating under the sun (photosynthesis is less efficient under too much heat[1]).
The fact that these "needles" also create a superhydrophobic effect would just be a side effect.