How did you know that's the chip being used?
Anyway, the actual input current isn't really the problem. It's the capacitance and inductance added to the circuit BY each input and the fairly long cable.
On the CS-line it might not matter because you can allow the voltage a couple of micro seconds to rise and fall but on the CLK and DATA pins it could when the clock frequency of the bus goes up (the chip you linked to is capable of 10MHz clock - not that you have to drive it that fast though).

The output driver in the PIC will have to charge and discharge the capacitance presented to it by all the inputs and the long cable.

By adding a line driver capable of sinking and sourcing more current than a PIC I/O to the CLK and DATA line the capacitance in the "external circuit" can be charged and discharged faster making the signals "cleaner". It will also add some protection.

All I'm saying is that it might not work to simply slap 50ft of cable on the outputs and then 50-100 chips along the length of that cable.

/Henrik.