The 2 battery packs were originally for balance,why use dead weight when an extra battery will do the trick?But the idea is not just extra flight time,it is for extra safety.Let me find the link........

http://www.rcbatteryclinic.com/parallel.html

It sounded like a good idea at the time.But like I said,If I have to add more weight to the tail to get an aft cg,I will probably remove the extra battery to make up the difference in added weight.