It's Right There! USE IT!!

Left 4 Dead 1 & 2 Group

Sprinting forward carelessly and taking massive amounts of damage while completely ignoring your team mates and ultimately failing miserably is fine for Versus mode, but when playing Campaign (Especially on Expert/Realism), being careful and paying attention to your team and surroundings is what will allow you and your team to succeed.

If you believe the Walk-Button is a useful tool, and not just a rough suggestion, then feel free to join.