Well, I'll tell you what happens and then you can decide how bad concentrate is for you. I'll discuss orange juice but the method remains largely the same for all concentrated juices.
First of all, you need to understand that pure, pressed juice is costly to transport from the various countries around the world where the fruit occurs. So someone had the bright idea of removing the natural water in the orange juice by effectively boiling it off and then shipping the concentrated juice to the countries where it intended for sale. However, there was then the issue of the fresh juice becoming perishable during freight and again it was decided that the concentrated juice could be frozen into huge blocks for transportation.
So what happens at the other end? Well pure, unconcentrated orange juice has a more or less constant specific gravity.The importer allows the concentrate to melt in tanks and they then add purified water to the concentrate until it reaches the specific gravity of the pure juice taken from the orange tree. In theory, you will then have the same quanity of water added in to the concentrate as was boiled-off in the country of origin. Products made in this way have to be labelled "from concentrate".
The problem over the years is that some companies in the UK have been rather unscrupulous in the way that they add the water back. A good few years ago, I remember one processor being caught red handed adding tap water to the concentrate via a hosepipe.
The ethics of this method have been subject to debate for years, but this is why orange juice remains a fairly cheap commodity. Juice from concentrate is also pasteurised at either end of the supply chain, but again this pasteurisation can be detected by aficionados and doesn't appeal to everyone.
On a lighter note, when I was a lad many years ago, you could buy the frozen concentrate in a tube approximately around the size of the cardboard in a toilet roll. The ends of the tube had metal caps and the idea was that you opened the tube, placed the frozen orange concentrate in a suitable container to melt and then added a set amount of tap water. After stirring, you had orange juice of standard consistency.
Orange juice that's not marked "from concentrate" is imported as it is following filtration. The extra volume in freight costs is passed on to the consumer and that's why it's more expensive and can only be found in the chilled cabinets in shops