according to this combat log parser, (some?) proc relics don't generate any threat at all: http://swtor.askmrrobot.com/combatlo...=0,f=2,t=2,b=1
(notice the Grav Wave row; 11642 damage and 0 threat).
So either the parser output is not correct, or such relics are pretty good to keep threat 2 % lower than other relics with similar effectiveness!
The other consequence would be that they are not suitable as a fix for tanks with aggro problems.
The relic used was the Dread Guard Relic of the Kinetic Tempest.
I don't know for sure how to read threat from the raw log, but it's possible that threat is the number in <such brackets> at the end of the line, and in that case Mr. Robot would be right about Grav Wave generating no threat.
I know that many groups have problems with threat in ops, and I think one of the reasons is that the difference between Rakata and Dread Guard results in much higher DPS (and threat) for the damage dealers, while for tanks a significant part of the improvement consist of extra defensive stats. If damage dealers would use proc relics, it would reduce the problem a little bit, without sacrificing much (if any) DPS.