I guess I wouldn't be looking at it as the cost of another battery pack, or the weight savings of not having to haul around another battery pack.
I am looking at it as the convenience of a single battery chemistry. I can usually charge the two 2300 A123 in 20 minutes, but the 1700 mAh NiCad pack takes a few hours. By using A123 batteries with a 2.4 system, I can eliminate the need for a second charger, and I can be charged up and ready to fly for the day in 20 minutes or less.

Greg