I use tap as well, though I run Purigen (though just passive in the sump). I just don't have the room to run a ro/di unit (plus I'm too lazy to set it up). I don't have any algae or other problems (though I'm not keeping SPS - because I don't have room to run a Ca reactor, plus I'm too lazy to set one up). In fact I can't grow macro algae in my refugium (perhaps iron deficiency? I was actually hoping tapwater would supply the iron or give me some phosphates or something).
In any event, can one use tap and run a successful tank? Yes. But I live in downtown Houston so I don't know whether some crazy well water/suburban water would have worse stuff in it. Of course if you want a ro/di unit then by all means, buy one and use it. Ro/di units, like many other pieces of equipment, cannot substitute for knowing how to keep aquariums (which comes from reading books/articles, experience, observing your livestock, buying healthy livestock, common sense, etc. etc. etc.)