I agree with everything here, and glad to see that FishyDP brought up the number one thing. Chlorine.....
@Dub,
I may have misinterpreted your post so I respectfully apologize and don't mean to offend.
Are you saying Dechlorinated water is the same as RO/DI water?
Dechlorinated water is simply that Dechlorinated only. The retail chemical products used to dechlorinate do nothing for TDS. Yes some will buffer PH but will not remove TDS.
RO/DI systems not only remove Chlorines but most everything else especially the unwanted TDS.