To be completely honest if you play a wammy aka with spirit speak, corpse skin is going to last a lot of time, so unless you can effectively spam AI, you ll always be better doing with it than without. Obviously after that you re going to use LS/AI as much as you can, but LS is not a guaranteed crit as far as I know. If irk's minimal resist is 50 fire and you use corpse skin, provided your wep is 100% of that damage type (or consecrus), you are going to do 30% more damage per hit. If 40% of your hits are non crits, the difference of damage will be significant (+12% of total damage). Of course, that strategy is only valid if your target lowest resist is fire of poison. If you wear FC items, casting Corpse is about as fast as one swing, so at worse you ll lose one LS every 40sec-one minute...To get enough spirit speak to make corpse skin and curse weapon really viable, you need to drop either anatomy or parry, and either way it pretty much balances out considering you either do less damage naturally or take more damage due to getting hit more. And while casting you'll either A. be getting interrupted or B. be waiting for it to cast (which means, not swinging therefore not hitting)
In short, lol corpse skin warriors
But that s all about the template you re playing, if you play pure sampire obviously it won t work.