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#20
I think there is a historic reason why the OS boot partition is still installed on a primary partition. Before Windows7, there was no hidden system partition which is the active partition containing the bootmgr.
In e.g. Vista the boot partition and the active partition were the same - namely C:. Thus C: DID contain the bootmgr (as active partition) and an active partition has to be a primary.
If you install Windows7 to a predefined primary, active partition, the installer will not create the 100MB seperate active partition. In this case you could not change that partition from primary to logical.
Next question would be why do we need a seperate 100MB active partition. But that is another long story.