But past a certain point, the parallel thing doesn't really make sense, unless you are doing something like endurance racing where you are prohibited from swapping the battery...

If the battery is perfectly cool after the run, let's say a 5000mAH LiPo, just swap it out and keep running. Running them both in parallel is only going to add weight.

One other thing to be REAL careful of is if you are going to connect two LiPo in parallel on-the-fly, make real certain that they are at the same charge level. If one is fully charged and the other is somewhat discharged, one is going to charge the other real fast and you could end up with a bad situation on your hands.