Most ports of 8051 follow a special quasi-bidirectional topology, so that the port can be used as input and output. But there are some disadvantages with that method. When being used as input current is flowing out of the port when pulling down the port line by external circuitry. More, this current isn't constant due to the switching between weak and very weak internal active pull-up. A true open drain output without any internal pull-up would avoid any such currents and offer a true high impedance CMOS input.

Anohter particularity of bidirectional topology is, that when toggling from low to high a strong internal active pull-up is turned-on during two oscillator periods to speed up the edge. So, the bidirectional port might not be used in a classic open drain circuitry expecting a true open drain output.

Kai Klaas