No matter how you spin it, you have to admit we're a pretty symmetrical species, and for whatever reason, nature seems to like symmetry. We're even programmed to find it attractive in facial features and whatnot. As for kidneys...
- 2 is better than 1: it allows for more rapid turnover of body fluid, greater clearance of chemicals, and reliance on a backup should one fail due to age/disease/trauma. Evolutionarily, this makes a whole lot of sense.
- embryologic origin: in the beginning stages of development, you're basically a tube, and you develop in segments. Each half of a segment receives the same growth signals for particular organs like lungs, kidneys, gonads, eyes, etc, so naturally you grow two* of each.
I think the idea is mainly that while you may only need one, having two is a definite advantage, as it allows sharing of functional load. People who are kidney donors/recipients suffer the unfortunate consequence that in future medical treatments, they are poor candidates for any drug that relies heavily on renal filtration and disposal, as the resulting nephrotoxicity can demolish their single functioning kidney. A patient with two kidneys would be able to tolerate such treatments with far better outcomes, giving them the edge for survival of certain conditions wherein a drug that relies on an alternate elimination route--for instance, the liver--may not have as good of a therapeutic index.
* Organs such as the heart and liver obviously contradict this, but in their origins, they are centrally located and extremely symmetrical, losing this property only in later weeks when more complicated signaling mechanisms for controlled apoptosis, positional change, etc. kick in.
