Depends... watching the action can teach a lot about the game, particularly football, hockey, or anything else that an injury or something can go unnoticed just by watching a box score. Basketball and baseball are easier to watch on Sportsline than say football is, because short of the quarterback and the running back, can you tell if someone's injured just by watching a score tick across the screen? Not that in the NFL that injury won't be brought to light immediately the next week, but in the AFL, injuries aren't reported very well, thus following the games play by play always helps, particularly watching the game live.
But yes, for baseball and basketball, I'm always watching too... I try to force myself to study while watching games online if nothing's on TV. Passes the time faster and I find myself retaining more information when I'm studying while watching Sportsline go by... funny how that works.