I wouldn't sub them completely, since if you do you don't get to clean. Just alternate them.
I would assume that you include the power clean in the program instead of the snatch because of technique concerns. Would you recommend someone with exposure to oly lifts and is following the program simply to increase their numbers to sub snatches for the clean? Or do you think a certain weight should be hit first, i.e. being able to perform a body weight power clean before advancing?
I wouldn't sub them completely, since if you do you don't get to clean. Just alternate them.
There is nothing about snatches that requires that you be able to clean any particular weight before you learn them. How do you think junior Olympic weightlifters start?
Senor Rippetoe, what do you think about this in regards to power snatches:
It seems like they'd be less beneficial for novices than power cleans because proper form on a power snatch requires that the bar travel through a much longer range of motion, meaning that much lighter weights need to be used.