It'll be fine for a while, and then your tank will start being overrun with algae, choking out the fish and the corals---even fish will start having trouble; and then you'll be stripping everything out to start over...or you'll have given up the hobby in despair because nothing works as advertised.
Here's the deal on ro/di: it is water which has been 'hollowed out', stripped of all content and minerals except the hydrogen and oxygen. It will quickly dissolve things it contacts---namely your ocean salt mix, thus providing a perfectly balanced real sea water.
Your tap water contains probably a hundred elements besides hydrogen and oxygen, or it would eat your pipes up. Our water near OKC even contained arsenic.
Evaporation concentrates anything the water brings in: you're constantly 'topping off' with ro/di [as well as using it in your original salt mix.] If there were anything 'brought in' with your water, it would stay in the tank as evaporation removes only the H and O. So a little arsenic could become a lot of arsenic over a year, piling up to a noticable dose.
That's why ro/di. It's expensive, but it's basic.
Get a good ro/di unit: After nearly 2 years use, I only had to recharge one cylinder and replace one, of the 4 mine has.