Modern mice and trackballs usually don't have rollers. They just have hard steel balls (about the size of a pinhead) that the ball skids on. In older mice, the rollers were not so much for support but to drive encoder disks (to tell which way the ball was turning on that axis). I can't think of any modern pointer devices that don't use optical sensing of the ball motion (if there is a ball at all); this is why the ball itself has a pattern of dots all over. Fully optical mice just track the relative motion of the mouse over the mousepad or tabletop. (They don't work well on featureless surfaces.)