I used to work at a place that made ultrapure DI water. I mean very clean water. As the safety guy I spent some time trying to find information about if it was safe to drink DI water or not. At the time, 2 years ago, I was not able to find anything good or bad. I had heard what has been said here that DI water is not good to drink, and the reasons, as stated above, sounded good to me. So I warned people not to drink it.
RO water while much cleaner than what comes out of the faucet should still have enough "stuff" in it that it is OK to drink.
I can't put it in terms of TDS but the RO water had a resistivity of about 1 Mega Ohm and the DI water had a resistivity of 18 Mega Ohm
Let the debate and flaming of me begin.
Scott