pic16f72 spi slave - ss pin function

[richard]

I'm trying to figure out a method of using the spi module in slave mode , but I don't quite get what function the ss pin has . the data sheet indicates that it resets the spi module (why ? what good does that do ?) .
I really want it to inform the program that a spi request needs to be answered (ie an interrupt) . I have used up all the interrupt pins and don't want to start again pcb wise and polling the pin seems inefficient and the chips have pretty limited resources 128 bytes ram 2k flash .I think I'm missing something here , has anybody got any tips on a good approach for an interrupt driven spi slave device.
I have got a whole bunch of these bourne absolute (8 bit grey scale) rotary encoders and a pile of pretty useless pic16f72's that combined might be useful.