I believe (not an expert on SW by any means) that RO or RO/DI water is often used in SW because the minerals are in the marine salt used. It is correct that RO and RO/DI water has all dissolved minerals and trace elements removed and is too pure to be used in the aquarium as fish absorb minerals and trace elements through a form of osmosis. You would need additives to use RO or RO/DI water in a FW aquarium.
Having 'said' that I have read that water in the amazon, due to the massive rain fall, is very pure and on the acidic side. With this in mind, depending on the tap water, one could make a case that very good and pure aquarium water just might be had by mixing tap water 50/50 with RO water. This maybe especially beneficial for some fish (e.g. discus?)
(I've just started doing this somewhat, except I'm using water I'm collecting from my basement dehumidifier instead of RO.)
On the other hand, many of the fish we purchase are tank raised using tap water so going to great lengths to adjust our water may simply not be worth the cost or effort.
I think the bottom line is, as others have mentioned, the use of good tap water, conditioned as/if necessary, will likely yield the best success.