You have saddled me up on one of my favourite hobby-horses, Erik - i.e. that the main reason for opting for a good hi-res sensor is not to be able to produce massive prints (Which should, in any case, be viewed from a reasonable distance) but, rather, to obtain much more data for processing.
If I print an A3+ print from the full frame of my D800 or D800E (36Mp) and compare it with a similar print from my D3s (12Mp), then there is no discernible difference in print quality. If I do an A3+ print from 10% of the full frame in each case, then I do start to see a difference. One of the ways in which I can use the 45Mb of lossless-compressed Raw file from the 36Mp sensor is to produce much tighter crops and still have printable resolution. But, of course, the other ways in which I can use all that data in the 45Mb Raw file are much more exciting and liberating than merely heavy cropping.