JaBa:
I believe R2 is only there to pull the gate of the MOSFET down so that the MOSFET can be switched off.
I just skimmed over the circuit diagram, and looks like if the opto's transistor is on very hard, then R2 and R3 make a sort of voltage divider, which can turn on the mosfet. When the opto's transistor is off, R3 pulls the gate to ground, or something like that.