I use AA as a balance between weight of battery and capacity of battery. As long as you're using it for a receiver battery (as opposed to running power equipment off of it), a cheap AA cell is providing 600mAh. For a radio that drains it at an average of less than 150 mA ... it'll give a good for 4 hours of use before needing to be recharged. I could use bigger batteries, but since I don't sail more than 4 hours, I have no need for bigger batteries that add only weight and cost to the system.

If I did need more sailing time or had more load, I still wouldn't need bigger batteries because I could use beefier AAs that are available pretty cheap with twice the capacity, so I could be sailing 8 hours or more on that size. I don't use smaller batteries because... well just because, I guess (no real reason there.) For the way I use them, I actually could use smaller batteries and save a little weight.

I don't use higher voltage because my radio gear is built for 4.8V. IF it would take higher voltage (and I don't know that it would even survive at 6V many systems will not), adding that cell to go to 6V means 5/4 as much voltage for 5/4 as much current flow and thus 4/5 as much time to discharge. I'd get no benefit from raising the voltage, but I would be risking my equipment AND cutting my sailing time by almost 25 minutes per charge.

So that's why I personally use a 4-cell AA pack.