6 wheel wide chassis - is there any advantage over just 4 wide?

[CrashTestPilot]

Is there any advantage to using 6 wheels in a wide tank drive configuation? Assiming you can fit 3 wheels on each short side. Would you even consider 6 wheel with center dropped in a wide configration? Sounds to me that the very short "wheelbase" created by the 4 wheels that are in contact with the ground would make the robot very tippy.

So what about turning effort in a wide chassis. If all wheels are in the same plane, would there be any difference in amount of scrubbing and turning effort between 4 wheel wide and 6 wheel wide. Assume wheel size to be the same.