You may know what you're doing, but odds are that you aren't reading what other players are doing... possibly you don't realize what certain moves look like from other classes, and it does take a bit to pick up on the animations, especially if it's a class you haven't tried yourself. If you find that you simply cannot move, your tactics may need a re-think.
I'd say sharks are there to keep players from shortcutting in maps or seeking some escape route without a penalty.