It increases threat generated by skills in determination stance and if you are a tanking warden its a must. Its the only trait a warden has that directly increases the threat we do. I don't have numbers with or without it traited simply because part of a tanks role is to hold aggro and you should always trait appropriately. The standard trait for a tank is 5blue and Terrible Visage. The last trait varies from a keen response or another in yellow to get the 2set bonus which i prefer. The reason players were not traiting Terrible Visage in U6 was because with it and using eob they were generating too much threat which was a bug and was going be reduced and is reduced now.
I know grinding for deeds is a pain but Terrible Visage is worth it. Its not impossible to tank without it but as they say "Every Little Helps"