We are currently working on a breakout-like game: Caromble!. At first we didn't have ball-ball collisions, but since shortly we've added it and we are very happy with it. It gives a 'surprise' effect to the player. It does make it a bit more difficult to play though, because it messes with your mind that was extrapolating the ball's position.
I'd say build it in, test it and see whether it makes sense in your game, given its difficulty and style.