the way I look at it, using RO/DI water with a TDS of 0 means you know exactly what you are putting into your tank, nothing. That is, pure water and nothing else. With non-zero TDS water you don't quite know what you are putting into your tank. Most likely a little bit of this and a little bit of that which may or may not be good for your tank. You just don't know :-)
Granted, you end up mixing your perfectly clean water with salt, but one would assume and hope that the salt that you are mixing into your water has been tested and is suitable for saltwater tanks and their inhabitants
Having said all that, there are plenty of people who use tap water with success.