I've thought about this subject a ton coming from a reefing background. In reef keeping, you absolutely never use tap water. You always top off with distilled or RO water and mix saltwater with the same. (I actually top off with distilled and use nothing but bottled seawater but that's a topic for another board)
With planted tanks, you always use tapwater. If you have something exceptionally wrong with your water or you want to keep discus, you may dilute it with RO but you still use tapwater.
This all makes perfect sense to me. Here's my (possibly absurd) thoughts on the matter:
The ocean is really really freaking big. It has the ability to dillute huge amounts of stuff down to miniscule, trace quantities. Rivers and lakes are much smaller and are in much greater relative contact with the ground. They're fed primarily by drops of water that run across the ground.
So, by my reasoning, saltwater aquariums should be nutrient poor whereas freshwater aquariums should be nutrient rich (comparatively speaking). RO for saltwater, tap for freshwater
I understand your desire to control every little component of the water and try to reach "perfect" parameters but I think it's probably misguided.
First off, I'm betting that even with RO, contaminents will make it in. You never really hit a TDS of 0 with RO water. Things besides water will be put into the tank too (gravel, fish food, fish, plants, fingers, the neighbor's cat).
Secondly, what are the "perfect" parameters? How do you figure out what they are? There are all sorts of stuff that you can't even test for in a hobby setting or in the trace amounts under which they are present.
What I do think it would be useful for is to experiment with the effects of different deficiencies and such. For example, set up a whole bunch of tanks, use R/O for all of them except a control group and dose them in different groups... See what effects it has on the plants and see if the effect can be reproduced time after time, etc. Someday when I have space for more tanks, I'd really like to do this for fun.