Why does the MacBook Air have five pins on the charger?
From MagSafe Wikipedia page

Pin 1 - Ground
Pin 2 - V+ @ 16.5 V DC
Pin 3 - Charge control pin
Pin 4 - V+ @ 16.5 V DC
Pin 5 - Ground


The inner large pins are V+ (16.5 VDC). Measuring with no load will give  6.86 V DC; the full 16.5 V is provided to the proper load.
The outer large pins are ground.
The tiny center pin is a charge control pin that assists with changing the LED color, as well as reporting the connected adapter type and serial number to the connected Mac.