The multiplier still applies even outside of a combo. Doesn't make it worthwhile, just getting the facts right. Rage of Halone and Butcher's Block can occasionally be worthwhile (e.g., right after a Provoke or an add spawn if you don't have a combo available and/or you need to avoid AOEing) because the higher multiplier is enough to give a little breathing room even with the lowered damage.
This is also why I only mark #1. I figure if I don't have threat locked on everything by the time #1 is dead, I need to tank harder. Helps keep me a little more alert.