The QCT format is how, historically, MM maps were distributed.
There is a vast amount of QCT maps floating around. MM were/are selling the 1:25k, 1:50k Ordnance Survey maps, road maps, some city street maps, some (rather poor) European road maps, and the 1:250k, 1:500k CAA maps and also airport diagrams.
About a year ago, MM moved to a QC3 format which is locked to the hardware and cannot be copied (till somebody develops a crack) but it doesn't matter much for now because one can go hiking etc with maps which are several years old.
The UK CAA maps moved to QC3 but some enterprising people have scanned all 3 maps, joined them into one, and converted the result to QCT
MM cannot stop people scanning paper maps and generating a QCT version. However, I think Oziexplorer is a better platform for a moving map GPS. I suspect MM will eventually collapse, once Ordnance Survey will terminate their monopoly.