Sadly, it is not SAR that is Nimrod's "raison d'etre"
Agreed, but I'm not sure that anyone is trying to say that it is. It will become apparent though, that the disappearance of the MR2 from the SAR 'toolbox' will have an effect on the country's ability to protect those at sea. I am sure that other assets will bridge the gap in the meantime but you would find it difficult to argue that a cobbled-together solution is as good as, or efficient as the Nimrod can deliver.
As for the "raison d'etre" of which you speak, there is nothing in the cupboard to fill the gap in the same manner. Shout all you like about Merlins, Type 23's and SSN but they just can't do it.