The point is, there is nothing for Omen out yet and they have had ample time to get into WotLK beta and now the PTR.
Blizz has it's built in threat now and that is what is being tapped into by Diamond, ZThreat, etc.
Now if they decide to look at updating Omen, then that will be nice. But Threat 2.0 library is dead in WotLK as Blizz has taken the need for it away.
As to the accuracy of Diamond, I have had very little issue with it being accurate. It has not consumed resources like Titan. (Oh another rant - FuBar is not working correctly in WotLK either and I am beginning to lean over to titan now too)
So for those that are taking the effort to update your mods for the patch and WotLK, I applaud you. For those that are not, I am moving on to other mods that do work.