Yeah, the benefit of series connection in this case is to supply enough power to the motor without requiring crazy high current handling capability, not to increase flight times.
Unsolicited electronics lesson/rant time...
In a 700 size heli you're talking on the order of 4000 Watts of power sustained in hard 3D, maybe even more. If you were running that off 6S (22.2V) that would require current delivery of 180A (Power = Voltage * Current), 225A for peaks of 5000 Watts. While there are ESCs that will do that, they are expensive and big/heavy. Plus, you need to supply that current from batteries (higher truthful C ratings) and you need to pipe it around (bigger wires, bigger connectors, more difficult to solder and route without problems, etc.). A 5000mAh pack would require a C rating of 36 (peak 45). While there are packs out there rated for this and beyond, there is some question as to the validity of those claims (highly recommend listening to the interview with the ProgressiveRC honcho during this week's RC Heli Nation podcast to get the skinny on this aspect of the RC battery industry).
The solution? Series connect two 6S packs to form 12S. By increasing the voltage of the system, you reduce the current requirements. Current for a 4000W draw at 12S (44.4V) is 90A. 112A at peaks of 5000W draw. Now you can go with a decent ESC with reasonable current handling capability, use 5mm bullet and equivalent connectors, reasonably sized wiring, etc.
Another benefit is that ESC heating is reduced. Generally speaking higher current is worse for heat buildup than higher voltage. Put another way, it is easier/less-costly to build high efficiency circuits using high voltage as opposed to high efficiency high current circuits.
