Background scounds distract your from whats really going on. You might hear gunfire and think someone shot the wall by accident, only to move away from the real threat. Special effects can be told apart from the real thing and look cool.
A neat effect I just thought of... Birds chirping! Bird sounds would be fun AND functional if they went away when someone made too much noise (e.g., firing a gun or running). Would be cool to walk into a forest and know if someone was nearby because the "auto-chirp-restarter" hadn't triggered yet -Matt