Hi Joop,
Like the 74HC595, the PCF8574 will not remember the current settings after a power on, and it would not be affected by a reset of the Netduino.
Unlike the 74HC595, when the Netduino comes out of reset, it could poll the PCF8574 to check the current levels on the I/O pins.
If I were using one, I would still write a class to control it for me. The class would include a method to read back the most recent settings without having to wait for an I2C operation to complete.
Have Fun - Paul