Mr. Nitschke notes that the added weight of an AWD system is a major reason for them not going this route. That’s certainly a good reason, but I imagine most people would be concerned about the change in handling characteristics from the switch to AWD over the weight. As effective as AWD is at increasing performance, it would strip away the M5′s sporty handling characteristics. In addition to making power sliding more difficult, AWD would sap steering feel from the front tires. By any measure, the current differential in the M5 is adequate for most conditions and weather scenarios.
The question is whether or not rear-wheel-drive will be enough for the increasing power levels of these cars. Mercedes swapped over because AWD made sense for the amount of torque they were making. You can make all the power in the world, but it doesn’t matter if you can’t get it to the ground. Evidently, Nitschke doesn’t think this’ll be an issue for the M models. Only time will tell if he’s right.