Boiled juice would taste like, well boiled juice. I wouldn't take some nice juice and concentrate it like that! If I had decent juice, I'd just freeze it as is and use it. Boiling fruit changes the flavor of it- think of the difference between a fresh apple and the apple in an apple pie. Or in fresh strawberries vs. canned strawberries. A "cooked fruit" flavor is unpleasant in most wines, I'd think.
However, I do boil bananas for my banana wine!