Pokèmon with Dream World abilities happen to be available via Dream World or via event. Some of these pokes are not present in Dream World yet (for example Volcarona), while others can be caught (Dratini needs 10,000 points before you can try to find it in the Island of Dreams). In other cases (i.e. Infernape), some pokes got released, but only in male form. It's to be noted that only female pokes with DW abilities can pass it (with 30% chance) to eggs.
For this reason, some DW abilties are "fully released", some others are still banned while others have some constrictrions and can't learn Egg moves or pre-evo moves.
With BW2, a new area in DW got opened and many new pokes could be used with the dream world ability. These releases can worth nothing (like Light Metal Metang) or be game-breaking (Technician Breloom or Multiscale Dragonite).
Hope you understood.