Demo Roar puts out a little threat, but not a noticeable amount. At 45, your tanking "rotation" is to keep demo up, use FFF every 6 sec, and spam swipe. Mangle at 50, Lacerate at 66(?).
Also, swipe passes lacerate in single target TPS fairly early. Unbuffed, I have a little over 5k AP and Rawr still ranks Swipe higher than Lacerate for instant TPS. The only reason to use Lacerate is to refresh the DoT. Also, keeping a bleed on the target increases the damage from Maul through Rend and Tear.
Okay, just tweaked gear on Rawr for a few minutes. It looks like the break even point is around 4800 AP. Sooner if you have more ArP or a warrior keeping up Sunders. This is because Lacerate's instant damage is a bleed (bypasses armor), while swipe is a physical attack mitigated by armor.