This topic comes up enough that I think it is worth continuing it here.
After seeing a post on Tank Spot that Dancing Rune Weapon provided threat, I tested it and found that it does indeed provide threat. I used Omen to do the testing since it provides the actual threat you have on a unit in the game. It isn't a calculated or estimated amount by an add-on. It uses the WoW API to retrieve the true threat value.
When you activate DRW, you will get threat equal to the damage it does. It does not get the threat multiplier for frost presence. Therefore, the total threat is only the damage it does. Since the DRW does 50% of your damage for 12 seconds, it should provide a boost of approximately 25% threat for 12 seconds every 1.5 minutes.
One of the interesting things I found out while testing it is that DRW auto-attacks on its own. It is not using your auto-attacks for its damage. If you cast DRW on a mob at range or run away after casting it, it will continue to attack the mob and do damage and provide you threat. This may have some interesting and fun applications.
Salted from DKi also created a video showing this effect. He established some threat on a mob and a druid then taunted it off him and then stopped attacking. When he cast DRW on the mob, it then switched back to him. Further proof that DRW provides threat.
Here is his video:
This testing was done on 3.3.2 and it still is true on the PTR so it should remain in 3.3.3.
I figured it buffed your Threat by 25% when you used it, so if you do... 8K TPS 1 quarter of that would be 2K bringing you up to 10K TPS wouldn't it? And yeah nice DPS boost, if you think you won't need Indestructible Potions you could even pop a Haste Potion with it.
Edit: On second thought think I got something wrong in my calculation... Nvm.
A Wise Sage once said: GearScore has more to do with Skill than you think... The more you care about it, the less you have.
I wouldn't dismiss the aggro-gain as minor. Rune Strike gets mirrored, too. Omen jumps to nearly 20k when 2 Rune Strikes crit at once
It's quite noticeable and the 1:30min cooldowns is great for most tank swap fights. Involving it into the pull rotation is a bit more troublesome, except if you use Empower Rune Weapon: IT > PS > DS > HS > HS > ERW > DRW > HSx6
As with dps rotations you want to summon DRW just before you are about to reapply your diseases. If you do not then the DRW you summon will mirror your strikes but its diseases will not be active so you will loose a large amount of its threat potential.
Another way to look at it would be if you're running dual tank specs then having DRW makes the build closer to a DPS spec. You can't afford to jump into other trees for DPS talents but for one talent point you get a DPS boost and some burst threat. If you ever need to throw on some DPS gear and DPS occasionally it could provide value then.
I've been using DRW for a long time now. When I didn't have an AOE offspec I often used it on aoe trash packs for that extra punch and threat from the DRW's diseases and blood boils.
I've never really quite figured something out though: when your DRW has diseases running on the mob with time left on them while the DRW expires, do the diseases disappear along with it or do they last for their original time? My Unit Frames only show my own active debuffs and the moment that DRW expires the debuffs disappear. If it doesn't actually fade away, it would be interesting if you could keep them up on a mob, since pestilence transfers all frost fevers and blood plagues on the mob..