Just to for a moment play devil's advocate, a TDS will flip out on your reef water, because the reef has a ton of Ions in solution. The calcium and Carbonate Ions we dump into our tanks like crazy, not to mention Na+ and Cl- etc etc etc....The problem is you can't control the good/bad ions from your tap. So your RO unit is pulling out all those wonderful calciums and sodiums that you are going to add later, but it ALSO pulls out many OTHER undesireable ones like silica and copper lead mercury or whatever. So hypothetically one could have a high TDS, and also have water fantastically suited for a reef tank, in theory. It just so happens, most of us don't have that kind of luck
For me, I use my RO because it creates a baseline (often times the quality of local water supplies varies drastically depending upon season/weather/etc) that I can then add supplements to, providing a more stable reef ecosystem..of course, if you aren't changing those filters religiously....
Cheers!