I had a lengthy discussion a year or so ago with bidulpik (the developer here) about this and while he didn't think it was a bad idea, he decided not to move with it because online he felt it would slow the game down because of waiting for the next player to decide whether to double or not. I have ideas which would, I think, get over that problem and I'm happy to outline them if there's wider interest in the idea.
The other issue is that it wouldn't fit the present way that backgammon games here are only single games - you could scarcely award double ELO ranking points after a doubled game! But we could make it possible to choose from several options - say the present single game without the cube, or a series of games with the cube, first to 3 or 5... points. Most other games on the site have options which the creator chooses, and a game with several rounds would be more a test of skill rather than the present one-off games which can come down hugely to lucky dice.
So - what do you think?
* In case you're unsure what I mean, there's a cube like a dice but marked 2-4-8-...-64. Before you throw the dice, if you think you're winning you can offer to your opponent to double the value of the game; if they say OK then they take possession of the cube (at its new, doubled value) and only they can double it next time. The catch is that if they say "No" then they've conceded the game at it's previous value! If they accept it then whoever wins that game gets the doubled number of points.