I guess my feeling is: why risk it? I invest in a fish to have it long term. I then go to the extra effort of TTM and then QT and usually two rounds of Prazi. If I'm going to make all of that effort, why wouldn't I also use a few extra gallons of ro/di water?
And if, as I think phc567 was suggesting, I am unlucky enough to need to treat with something (like copper), I do not have to worry quite so much about anything that might be in my water to react with my treatment.
Sure, well water eats through resin, but ro/di filters are by far one of the cheaper aspects of this hobby.
To me it just makes good sense to use the best water I have available all of the time.