Hi Mike,
You must have missed app note 941A
Part of the reason for using a gate resistor (Rg in series with the gate) is due to the small inductance in the source, which as im sure you know increases with the circuit layout. This is of concern when paralleling MOSFETs but could be a concern for single devices too. The source inductance acts as a counter voltage to the gate (turn on) voltage, which could cause the MOSFET to turn on, turn off, turn on, turn off, etc., which of course is not a good idea for a number of reasons. Maybe we can think of this as the 'Miller' inductance
Now in any circuit that contains inductance we can damp out oscillations by simply adding a series resistor, but that usually causes a decrease in efficiency so we try to minimize that while still getting a more well behaved circuit out of the deal.
In a perfect world with perfect components and no stray capacitance or inductance we would want to use a pure voltage source to drive the gate so we can turn on as fast as possible and thus reduce switching power losses. In this world however that doesnt happen because we get side effects that we have to *also* deal with, and that leads us down paths that may cause a slight deterioration in our perfect world design. It's like anything else, when we try to get good performance we many times have to do something that has negative side effects to make the whole thing work in the real world.
To investigate this further, you could try to find the notes that talk about the actual switching waveforms that occur with various types of loads and those values. Those notes include an Rg value too. Otherwise you might try some simpler circuit simulation with a perfect voltage controlled switch and some 'source' lead inductance and see what happens...you'll also have to add gate and drain capacitances too i guess.
In short, the MOSFET is a very high gain device that requires special techniques to control. Think about trying to control a switching regulator circuit with a *pure* inductance (no series resistance) and pure capacitance, keeping in mind that even a small amount of energy into the circuit will cause the LC to oscillate forever. Then add even a small series resistance and see how much more easily it is controlled.