The big benefit of the SS is the pulserate, especially whe you are talking 2000 plus line encoders which is common on the industrial orientated servos, heck the spindle motors I have on the mill and lathe have intelligent encoders which can be set to 2 million counts per rev, even the SS wouldnt be much use there
The pulse from the parallel port will vary from computer to computer, some will be very nice others not so nice. I have scoped most of the computers ports I have ever used with Mach but the SS is in a class of its own even compared to the best PP pulse. Thats not to say the PP is no use, it usually does a very good job.
Hood