[KrazyCarl92]

There would still be an advantage to chaining the rear wheels with the center wheels in that in the case where the rear wheels are elevated, the front and center wheels would still be powered by 2 motors instead of just 1, allowing for double the power in this case.

This could also be the case if the normal force is unevenly distributed among the wheels, e.g. center of gravity is more toward the front of the robot or the center wheels are dropped. This would mean the rear wheels would have less load on them because there is a lower frictional force on them, so you would get more out of your motors by chaining the wheels together.

Worth noting that these two reasons are both conditional, however.