Turning ability.
In any skid-steer drive with more than two wheels, a phenomenon known as turning scrub arises. Wheels are dragged sideways as the robot rotates, creating significant frictional resistance to turning. This resistance can be large enough to completely prevent the robot from turning at all. Typically, this point is reached if the wheelbase is significantly longer than it is wide, as is the case in a typical, long orientation four wheel drive.
There are a number of ways to reduce turning scrub within a four wheel drive, none ideal. Casters, slick wheels, omniwheels, and unpowered wheels reduce tractive force, and shift the robot's pivot point. Shifting the robot's center of gravity to one end or the other can make it unstable. Wide orientation drives don't work with every robot design. Shifting wheels closer to the center of the robot makes the robot tip-prone.
Drop center 6 wheel drive takes full advantage of a robot's 38"x28" footprint, does not sacrifice traction, and reduces turning scrub. Since only four wheels ever contact the ground at once, the wheelbase at any given time is short and wide. But the presence of 6 wheels, spanning the length of the robot, gives the robot great stability.
For more on the phenomenon of turning scrub, I highly recommend this white paper:
http://www.chiefdelphi.com/media/papers/1443