ive seen plac breaking ToF yes, BUT also seen it not, seems to have the same %chance as all 'hits' wether they do dmg or not...
but imo dark shoulda stayed on scraps, the only thing they got tahts good is ToF, everything else hardly hits anything and therefore makes u depend more on secondary :/
Midnight Grasp critical is scary damage + Dark is one of the least resisted Damage Types (always useful)
The ToF/Placate combo is VERY useful. Placate does not break ToF, but it will still have it´s effect.
So, the feared opponent will do his "Huh? What?" animation and resume trembling afterwards (except if the fear duration is over of course). This way he will not run around but will patiently wait for you to AS him.
If it has eyes, you can blind it, if it has blood, you can make it bleed, if it has a mouth, you can make it scream.
TOF rocks for PVP use BU/TOF to open with my Dark/Reg all the time then placate AS shadow punch should kill most squishys and the TOF roots them so stops there annoying running about 
one of the things been debating is possibley doing a dark/ stalker, questions i have is pvp wise, if you hit a target with TOF, does placate break the TOF or not, thinking being able to go BU&AS, TOF, placate etc be a nasty combo but not sure about it as i dont have a dark/ stalker was also thinking dark/dark might be good for pvp, know several of the PAoe stuff in /dark is hide breaking but was thinking kicking in the AOE fear jsut after hitting something with AS be very handy for surviving lol