Games could serve multiple purposes: out of band communications (i.e. not phone, email, or mail), rehearsals, and recreation. Since the Caliphate is going to be a while in coming they have some time to kill.
I don't think there is any surprise that WoW or similar games would have broad appeal, even among terrorists. After all, the Harry Potter books have been among the most popular reading for inmates at Guantanamo Bay.
What Prisoners Are Reading at Gitmo
... Harry Potter. He may not come riding in on the back of a hippogriff to free his favorite captives from their own version of Azkaban, but he shows up once a week on a cart of books from the prison library, offering an escape of the imagination treasured by many. Indeed, the Harry Potter series has been one the most popular titles among the 18,000 books, magazines, DVDs and newspapers on offer from the prison library at Guantánamo.
Other offerings in the library started in 2003 include the The Lord of the Rings trilogy, the Twilight series and a self-help book called Don't Be Sad. Prisoners don't browse the shelves of this particular library; instead, they wait for a weekly visit by a cart of books prison officers think they might be interested in. There are mysteries and books of poems, copies of National Geographic magazine (a favorite), dictionaries and science textbooks. If the prisoners see something they like they are allowed to check it out for 30 days.
The library's offerings now span some 18 languages including Arabic, Farsi, Urdu, Pashto, Russian, French and English. Officers scan newspapers to stay up on the latest titles and try to meet requests from prisoners — though finding books in their native languages can sometimes be a challenge. "I tell ya, Dan Brown's been beating me up lately," says Navy Lt. Robert Collett, who as the officer-in-charge of detainee programs, is known as 'Dean of Gitmo U'. "All his books are very popular, but we don't have all of them in Arabic." When the military has trouble finding a title in a certain language, the International Committee of the Red Cross (ICRC) sometimes steps in. Martin De Boer, ICRC's deputy head of the regional delegation in D.C., says his group sometimes sends its representatives in far-flung places to local stores in order to answer requests for novels in Uzbek or magazines in Bahasa (the language of Indonesia). "Access to books and news from the outside is very important to the prisoners mental state," says De Boer.