While a fish may survive on nothing but nori, fed exclusively it certainly will not fulfill all of their dietary requirements.
While most Tangs are indeed primarily herbivorous, many of them also take in numerous small invertebrates, which in certain species most likely provides a significant portion of their protein intake in the wild. There are essential amino and fatty acids that are present in marine proteins that are not present in tissue from terrestrial plants or animals. Most fish also require long chain fatty acids (C20 and C22) that are also not found in tissue from terrestrial organisms, and this would include most species that have been classified as herbivores. (both freshwater as well as marine)
Even more importantly, in the wild these fish graze continuously all day long on these various algae species, and the invertebrates found within them, which is certainly something that does not take place in most aquarium settings. In other words, in the wild they have a constant source of varied nutrition, from sun up, to sun down, and there's a never ending supply of it. The reason for the long digestive tract, is that the bulk of their diet is of low nutritional value, hence the need to break it down fully before it passes as waste. If they didn't, they would pass most of the nutrients before they could be fully assimilated.
While the various species of Porphyra do indeed contain numerous vitamins & macronutrients, and can provide a certain amount of high quality nutrition, I don't believe that you will find a single paper or study on this subject that involves herbivorous species such as Tangs, that states that nori is in & of itself a complete food that meets all of their dietary requirements.
You might find the following two links interesting reads.
http://www.flseagrant.org/program_ar...tlas/index.htm
http://fishweb.ifas.ufl.edu/Faculty%...urgeonfish.pdf
BTW - I know what the professor involved in the studies linked to above feeds all of their incoming marine fish while they are in quarantine, including their Tangs, and it isn't nori.