I could be wrong on this...
But I was thinking that rain water is generally high in nitrates. It is this way because of where it is absorbed from and where it touches/mixes before it falls.
Nitrates are good for plants/flowers/crops. I say this because that is a major ingredient in miracle grow and alot of other plant foods.
That is why indoor house plants respond so well to rain water after a long time of only getting tap water.
I am not sure that i would want that in my tank just because of that. A low nitrate system is what people strive for... so... ?