I suppose we should look at why we ground services
250.4(A)
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
Those are not scenarios that are present with a portable generator. You are not driving a rod so you are not going to ground the system anyway. Bonding the neutral really serves no purpose.
If you are connecting to the house with switched neutral transfer equipment (taking the MBJ out of the circuit) you would need to bond the neutral in the generator but if the transfer equipment is only switching the ungrounded conductors you should leave the generator unbonded.
