I agree with ringi - the trains are grossly overcrowded at peak times, including the late afternoon trains out of Piccadilly, so it's reasonable to charge a peak-time fare then to encourage people (such as shoppers), who don't have to travel at that time, to come home earlier or later. But I don't see why the peak fare should be charged for a train going IN to Piccadilly at, say 5 pm, as that train will be almost empty.
The real issue, though, is that no-one should be paying a penny more for the disgraceful train service we get at Marple and Rose Hill. As marplerambler rightly said, we have been singled out to suffer the worst trains in the country - they are cramped, noisy, uncomfortable and unreliable. And yet this is one of the most highly subsidised services in the UK. Not surprising, I guess, as Northern Rail is such a laughably inefficient organisation - have you ever noticed the number of 'revenue protection' staff hanging around on the platforms at Piccadilly? Some days there are eight or nine of them, just standing around chatting to each other!