-Tohit on enemies acts exactly the same as defense with one exception: debuffs can be resisted. Against foes higher level than you, your tohit debuffs will be resisted. Also, AVs resist something like 85% of it.
Defense can be relied on more, but -tohit helps your entire team.
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don'T attempt to read tHis mEssaGe, And believe Me, it is not a codE.
For AV's I can most often plan to rely on +defense bonuses from my team, and If they aren't putting out, purple candy will probably do the trick. The more I think about this, the more I lean towards the road of the +resistance toggle. The fact that AVs resist ~85% of my tohit debuffs is a bit unsettling, but not surprising.
Thanks for the response.
I've always been a fan of layered mitigation myself, so I'd nudge you in the way of the resistance toggle too. When it comes down to AVs, not only should you have teammates giving you buffs, but someone else will likely have the aggro.
AVs resist more than just -tohit. It is a bit unsettling, but imagine how trivial AV fights would be when all it took was a single rad defender to floor his tohit chance. I like things the way they are.
Where to now?
Check out all my guides and fiction pieces on my blog.
The MFing Warshade | The Last Rule of Tanking | The Got Dam Mastermind
Everything Dark Armor | The Softcap
don'T attempt to read tHis mEssaGe, And believe Me, it is not a codE.
Layered defenses is a good way to go. Just bear in mind -tohit only works as effective defense once its applied to the enemies.
On a TF unless you always cornerpull with DN you'll likely still have to eat a volley or two depending on the circumstances. It's not a huge concern given how absolutely massive Fearsome Stare is, but do bear in mind that -tohit is more finicky than straight +def.
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I understand that I will have to work more to rely on my -tohit for defense, but it seems like that way will allow for more potential to soak damage.
My Purpose in making this thread was just to confirm that I am able to count on my -tohit.
Ah, well then, with the exception of AVs as Dechs mentioned, yes indeed you can 
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I have a few questions regarding Dark Miasma. Can I safely rely on my tohit debuffs to act as a large part of my defense (read: chance to dodge/avoid attacks)? I'm currently working on leveling a fire/dark corr, and I'm not sure which way to go for survivability. I see a choice between high resistances or soft-cap to smashing/lethal damage. I want to know which way will allow me to soak more damage.
If I pick up a Patron Armor that provides resistance (combined with tough, ~50% to smashing and lethal), I can still find over 20% defense to all damage types with Shadow Fall, set bonuses, and the Fighting pool. However, If I pick up Scorpion Shield, I can be soft-capped to s/l damage regardless of relying on my tohit debuffs.
So, assuming that Darkest Night will provide 23% tohit debuff, and I will have 20-24% defense, will the two combine to be equivalent to soft-capped defense? Will that make a resistance armor the better path? (I will also have fearsome stare providing about 22% tohit debuff).
Should you decide to answer, keep in mind, I plan to run a lot of level 50 SF/TFs.
Also note: the majority of my set bonuses will be devoted to recharge time (perma hasten).