I'm sort of having trouble coming up with an opinion for this one.
The tank in question is a nano? What sort of inhabitants?
The beauty of a nano is (well .. the beauty beyond the inhabitants, I mean) is it's simplicity. And simplicity implies being able to care for it without all kinds of gizmos and whatnots.
Ultimately it will probably depend a bit on what you hope to keep in there, but I think I am going to side on the lumpers on this one, i.e., you don't really need an RO or an RO/DI for a nano.
If you buy a unit though, it's not wasted money, it will never hurt to have it.
So I guess what I'm saying is, if you're looking for an excuse to buy a toy, go ahead and buy it. But if you're looking for an excuse to hold off and divert the funds to other things like lighting or skimmers or whatever ... then go ahead and buy it another day.
I look at pictures of my reef tank that I had 6 years ago and it astonishes me how nice it was. And it had no halides, a wee little CC skimmer, no sump, no calcium reactor, no kalk, no RO, just tap water. Ok, so it didn't have fuzzy coloured sticks, but the softies and the fish did just fine. In fact my BTA's looked nicer then than they ever do now. And with fewer things to go wrong on you, fewer things will go wrong. And I never did have the armageddon scenario that I was promised would happen because I didn't have all that extra stuff .... in fact all the disasters I've had while reefing since moving onto "bigger and better" has been because some piece of equipment failed or was taken offline temporarily or whatever. ... I'm not sure what my point is here, I think all I'm trying to say is that "more complicated setups" aren't necessarily "better." It's all in the eye of the beholder.
So basically getting back to your nano ... it's your tank so have fun with it. If it's mostly a softies type scenario then you're probably OK without the huge investments. But for more finicky corals then yes you may need to make the investments now (I expect you'll find that a nano will be outgrown quickly by a lot of those things anyhow).