I do believe Nokia has been very clear that the high megapixels is meant to be used as supersampling to reduce noise in normal sized pictures. They are after all targeting the enthusiast photographer who likes to (believe he) understands how camera's work.

Also, from an engineering point of view it's pretty neat: if the idea is to downsize the final picture to 5 mp, while using cropping on the pre-downsampled image to zoom, that means you can focus on having a high optical resolution in the centre of a fixed focal lens, and worry less about the edges. That has lots of advantages: a fixed focal lens is less complicated than a zoom lens, which has the side effect of making it easier to keep the lense flat,and getting a higher optical resolution in the centre is easier to do from an optics standpoint.