2GB of RAM does more than just make tabs/apps refresh less. The animations in iOS are actually cached in RAM. So that's why sometimes on an iPad Air 1 for example, the first time you open a folder in a long time, it's extremely laggy and stuttery. But after that, you can exit and open the folder over and over it and it will be a perfect 60fps.
The iPad Air 2's 2 GB of RAM allows for these animations to remain in memory for an extremely long time, and add in the incredible processing power with the A8X, everything is buttery smooth, every time.
2GB of RAM will definitely serve the iPhone 6S quite well. We have been on 1GB of RAM for far, far too long. We should have gotten to 2GB of RAM with the 5S because of its 64-bit architecture. The 64-bit system the A7 chip runs has larger instructions, so more RAM is used. 2GB of RAM was ESPECIALLY needed on iPad Air and iPad mini 2/3.