Hi,
That pin (Leg 4) is the Programming pin, when used with an external programmer. To use that Programming mode, the pin to taken to about +8 volts, so it cannot have the normal "electrostatic protection" diode to the Vdd (supply) rail. Theoretically, taking the pin above 5 volts could cause the Program (EEPROM) to become corrupted, but in practice the (re-)programming sequence is complex and unlikely to happen by accident.
Strictly, if this pin is used as a serial input from "TTL" levels, then adding a diode would not be necessary. But PICaxe is intended (to have the capability) to connect directly to "COM port RS232" voltage levels (possibly up to +/- 15 volts) via the 10k+22k programming resistor network. That network only works (with RS232 levels) if there is a diode from the input pin to the Vdd rail. Otherwise, a "high" RS232 level might destroy the chip (absolute max 9 volts rating on that pin).
Yes, I also considered it to be a rather "useless" pin, but it can be the "Timer1 Gate" hardware input (which can be useful) or used as a high impedance output by enabling its Weak Pullup resistor (PULLUP %xxxx , or I prefer POKESFR $8C,8).
Cheers, Alan.