This ability causes the same level of threat as the Voidwalker's Torment. The improvement in the Felguard's ability to hold aggro is in its high damage, and Intercept. Torment may cause more threat than Anguish due to the Improved Voidwalker Talent, for which the Felguard has no parallel.
Anguish's primary role is in solo play. It is useless in pvp, but can be useful in groups, as a plan B for when the tank dies. However, a Felguard cannot tank most trash or any bosses in raidinstances, and its threat per second (TPS) is negligible compared to a raid-geared player tank.