I have previously suggested the Sierra as an option to replace the remaining Blackhawks, currently used for special operations support, rather than as an additional type. This is because it had been suggested that the MRH90 had been found unsuited to the role and in need of modifications to be able to adequately conduct the required missions.
To me the Sierra offered a number of advantages to other types that may be considered, even over a modified MRH90. First is the fact that special operations support was one of the missions the Sierra was designed for, second it's commonality with the Romeo, then its full marinisation. In addition it would also add other capabilities to the mix other options likely would not, it is designed for CSAR (Combat Search And Rescue), it is fully armed (with systems also used by the Romeo), it is designed to be configured for mine countermeasures, as well as for maritime attack missions.
Overall I saw it as potentially an opportunity to not just replace an existing capability but to fill a number of holes in the ADF, as well as back up existing, stretched capabilities, at minimal outlay or risk