The primary advantage of 1xAA over 2xAA is the smaller size. 1xAA lights are obviously MUCH shorter. Also, 1xAA lights can be brighter than 2xAA lights. Consider that the Zebralight SC5 can put out 500 lumens, which is quite a bit brighted than the brightest 2xAA. This has nothing to do with any technical disadvantages with 2xAA, but rather that nobody wants to build a bright 2xAA (2xAA typically tops out at around 350-400 lumens). On the other hand, as you would expect, 1xAA lights are at a severe disadvantage compared to 2xAA lights when it comes to runtime, particularly when running the higher modes. And because the voltage of 2xAA in series is higher, 2xAA lights don't work your batteries as hard. This is advantageous is you plan to run your light on alkaline batteries.
Running a 14500 has a few advantages over running a AA (provided the light can run a 14500). Usually, a 1xAA light will be brighter on a 14500. And brightness will typically be better regulated with a 14500 than with a AA. Runtime can also be longer with a 14500 since driver losses are far lower (provided the energy storage is the same on both cells). Finally, 14500s are easier to charge when away from home. This is particularly true when charging in the backcountry with portable USB solar panels.