It would be helpful you to understand the reasons for the story and to understand the game mechanics, but revelations would probably would give a summary to go over major points of the story. So in short, unnecessary but recommended to play the previous games. Kinda like watching last Lord of the Rings before the first two.
"Ubisoft has released a "Previously On" trailer for Assassin's Creed in preparation for Revelations tomorrow. Check it out to catch up or refresh your memory" There is a trailer to help you understand what has happened in the past, so you don't have to play previous games... http://www.gamefly.com/news/article/26034/assassins-creed-revelations-previously-on-trailer-catches-you-up/