DV
You asked why the A. I pointed you in the right direction. Whilst the Nimrod's job, at present, does not qualify as attack, that's not to say the MRA4 role may be exactly the same as the MR2. Way back, when the contract was in its infancy, its fair to say that attack may have, and may will be in the future, an aspiration. Who can predict the world's affairs in a number of years. Almost impossible to say. Hence, there may be an aspiration to use it in that role, maybe not. It may be comforting to believe that there lies some forethought in this decision making.
Its also not unreasonable to understand that the world is a changed place since the contract was drawn up. But that's not to say MRA4 will not be involved in these roles you point out. Flexibility, etc, etc.