After Microsoft stopped making standalone batteries available for sale, my original batteries started to lose their maximum charge quite rapidly.
At that point, my only options were third party batteries, or to shell out $30 for a single battery with an extra charge cable. It should be noted that the third party battery I purchased does work with the Play n Charge cable, but does NOT work with the Quick Charge unit. I was very disappointed by this.
So as soon as I saw this new package for sale, I jumped at it! Now I can charge a spare battery or two in my Xbox 360 Quick Charge Kit
while I play one to death and have put the Play n Charge cable back to very rare useage when 3-4 players are playing and I don't have enough spare batteries ;)