According to Bulbapedia:
Some cards in sets that are part of the current rotation are reprints of cards that appeared in older rotations. If a card is included as part of any set in the current rotation, any version of that card is legal for tournament play. For example, at least one version of Rare Candy has been legal in every modified format since it was printed. Thus, older versions of Rare Candy, even those from rotated sets like EX Sandstorm and Great Encounters, are still legal cards. Similarly, basic Energy cards from all sets are still allowed in tournament play.
Okay, that I understand. Here's the catch, though. What about cards that have different names, but do the same thing? For instance, once Plasma Freeze gets rotated out, would Juniper still be usable considering Sycamore is still in the rotation and does the same thing? (Yes, I'm aware you can only use one of the two, no need to ask.)