"The crap that is in the water is in a crappy form for the plants to drink."

Not necessarily. Most of the crap in water is too large molecularly for the roots to allow to pass through the membrane, and almost every nutrient for plants (minus nitrogen) is water-soluble and already in an ideal solution. Calcium, magnesium, etc., are all pretty much water-soluble and bioavailable. Of course, these levels vary greatly by city and thus you must eventually figure out the levels between various nutrients. While most nutrients for hydroponics are balanced and buffered for use with RO water, many people find themselves using off-use solutions (Florikan Dynamite is what I used for flowering once in hydro, and it worked GREAT minus clogging up everything) and thus the "grab-everything" nature of RO water will cause massive swings depending upon what it 'grabs' and binds with.

Also, People using *JUST* RO water are doing it only slightly wrong. You have to pre-treat water before running it through RO. Pretreatment is important when working with RO and nanofilters due to the nature of their spiral wound design and picometer-thick meshes. The material is engineered in such a fashion as to allow only one-way flow through the system, and as such, the spiral design can't allow for backwashing with water or air friction to scour its surface and remove solids. Because built-up garbage cannot be removed from the filter surfaces, they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF system. Best bet - start with a loose Brita or Pur filter, then push it to RO.

Again, Your Mileage May Vary. If you've got excellent low-PPM water from the start, you have not much to worry about, and your pH should be stable through all stages of purification. If you have a bunch of salts ions in your water and the water has been buffered to neutral, the second you remove all of that garbage, you're going to have acidic water due to the ionic imbalance.