A little more lateral thinking is required here. Does Marple need a tram service to Manchester via Bredbury or Guide Bridge? The answer to this is no.
But you need to think about the line as less solely focused on serving Marple. Marple may not be crying out for trams, in your opinion, but stations such as Reddish North, Ryder Brow, Belle Vue, and so on are all massively underused because the service terminating at Piccadilly makes it worthless. Converting it to a tram-style service and running across the city centre, especially to somewhere like Salford Quays, would immediately bring a massive increase in custom from these stations. It's not all about us.
The report suggests that the majority of the 60% boost in passengers would come from these under-served inner city areas, as people are drawn by the frequency of the tram and the fact that it actually goes right through the city. For Marple, this would give the trickle-down effect that increased patronage from the inner-city areas means being able to justify a better, higher frequency service all the way along the line to cope with the new demand. We all benefit.
I agree the Stockport problem would be far better to solve, but perhaps we should see tram-train conversion of the existing line as the precursor to that. Stockport MBC appear to be (very slowly) progressing the case for a line from Stockport Bus Station, through Portwood and across to Bredbury, roughly following the old Tiviot Dale line/current M60. It'd be a rather expensive build, but having this ready-and-waiting converted tram-train line from Bredbury to Marple for it to connect onto could just move things along. I doubt otherwise a tram to Stockport would ever come before a tram-train to Manchester.
As it stands, the line to Manchester has to take priority because it's currently operated by life-expired diesel rolling stock that, as well as being clapped out, is going to be falling foul of new disability regulations come 2019. They could order new diesel units (not going to happen), or wait until better spare units become available from electrification projects elsewhere, or they could just go ahead and finally convert our line to tram operation - increasing income, removing the subsidy and ultimately saving taxpayers' money through a little investment.