SAROwl - do you mean to imply in your post that the Portland crews don't train because they don't have a spare? That's the way it reads.
Are your S61s a similar spec to the CHC SAR ones in Ireland? By that I mean that their SAR aircraft is SN501 fully autopilot equipped but that their spare is likely to be a non-autohover, non doppler aircraft (standard S61?).
If yours are of the same spec then you cannot say that there is 'little diminution of service' when transferring to the spare, especially for night over water work.