If the answer to the question "Do we need a long-range MPA with good ASW capability?" is yes (which it should be, given who we are, where we live and operate and who has submarines or could regenerate them quickly) then this project makes sense. Not very cost-effective sense certainly, but sense nonetheless.
There are no other options out there that are close to entering service. When the original competition was run ISTR the options were Nimrod rebuild, P3 rebuild, Atlantique 2 or hang around and wait for the P7 to materialise (it still hasn't). Given the long range & ToS requirements and fuselage volume required, that pretty much left Nimrod & P3 rebuilds. At the time, the P3 rebuild was probably the better bet in terms of export potential, but the decision was made and we have to live with it.
Despite the best efforts of certain people in town, the submarine threat is still there and MIOPS also require a wide area surveillance asset that in the absence of CV-based S3 vikings is best done by Nimrod.