Here's the backstory of my conclusion: so I'm questing in Pit of Iron when I have to kill the Devourer. Big stupid worm that seems to have nutty runspeed and hits like 12 dump trucks. It promptly kills me, and I get all depressed-like, figuring I'll give it one more shot before going out and retraiting lightning just to kill the thing.
Now, by this point I was used to dropping my rock on tougher pulls, both for the heals and because on multi-mob pulls I could usually get the rock to tank a guy for a few seconds until he got hit with an AoE. I also have the CD reduction on DF on my stone because I thought, hey, it'll be nice to toss a nice long DoT on several targets at once, right?
So then I get ready to pull this dumb worm again, thinking I better dump damage onto this thing as fast as possible to see if I can kill it before it kills me. So I put Fall to Flame on it, Armor of Flame on myself, then hit it with Essay -> FR -> FR then spam MFx6 to instantly hit it with SW. Hit it with SW, instant WoC, Essence of Flame, more FR....
The thing is, the worm actually de-aggros off of me about halfway through my MF spam and attacks the rock. It continues to attack the rock through the duration of SW and doesn't come back for me until the rock dies.
So after reading around a bit, especially this thread (specifically this post) here: http://forums.lotro.com/showthread.p...22#post6066522 and remembering some other silly bugs, it hit me: you know how tons of DoT's and HoT's and whatnot are double-ticking? As in, a pulse falls off the end of the duration but some other effect is front-loaded into the initial application? I know, for a long time on my champ, Red Haze was working this way. Instead of a pip at application and a pip every 15sec for 45 sec, it was just granting 2 pips off the bat and none over time.
So my theory is this: MF isn't a guaranteed threat reduction but instead lowers threat in an amount described in the post I just linked, making the skill threat positive for RK's above a certain mastery threshhold who maintain the DoT on their targets. However, if the threat-reducing component of the skill is bugged in the same way as so many other over-time effects, maybe it's dumping its threat-reducing elements into the skill application rather than during ticks, making spamming MF a very powerful threat reduction skill.
Now, my RK is yet but a new-to-75 weakling, but I want to do some more testing on this theory and thought others might be interested in doing the same. A good test might be to do something similar to my Devourer test but with a mob that hits less hard and has more morale (maybe some elites in Angmar, or shard droppers somewhere?).
Of course, even if I'm right, there are still limits to the potential of spamming MF as it'd be very power-hungry. That, plus well-geared RK's are going to be doing signficantly more damage than I, but overall I think if it's working the way I suspect it might be, and if you can spare the power, you might have something to gain by spamming MF a whole bunch during threat-sensitive parts of fights, like right at the start of a fight, or when the tank has to run away from Kalbak, or when the tank gets the B/P/E debuff from Bukot, etc.