Leaving aside 3D for the moment I cannot understand this Ultra-HD stuff really. I've seen digital projection of movies on large screens and although slightly higher than HDTV's 1080 resolution it was 'good enough'. I saw no problems with lack of clarity or visible lines/pixels on those projections.
So why do we need such a high resolution? HDTV was about making the image clearer and allow increased TV sizes without having an attack of the fuzzies. Plus in order to feel those benefits the medium itself needs to be clean and artefact free.
Why do I need my film grain for instance to be even sharper?
3D is where it's at. As 3D digital cinema projection becomes ever more popular they will need a way to translate it to the home viewing experience without those crappy red and cyan glasses.
Higher resolutions are not really needed other than for super duper large displays and even that is debatable. Waste of bandwidth.