You should use the Max232 driver. RS-232 signals are >+3V and <-3V. The 8051 is a TTL output, which can drive a load to >+3V, but can't drive it to <-3V, so it's not going to be heard by an RS-232 receiver. Also, RS-232 drivers are handy because they usually can take a dead short without frying. I doubt the 8051 outputs are so forgiving.
Another reason to use a driver is to defend the 8051 against accidental overvoltage.