So I just got this skill and at first it seemed too good to be true. A massive amp to some skills with a 5 second recast. I thought it was going to be a part of every rotation. But then I realized the cost is like 27% of your mp while Darkside is active making it significantly harder to maintain MP if you tried.

So I can see how it plays out like this:

Need health? Souleater+
Need aggro? Power Slash+
Need both? Abyssal Drain+ (Though I don't have this yet)
Need to dodge? Dark Dance+
Need to not take Magic damage? Dark Mind+
Need to blind? Dark Passenger+

So I can see how it's uses are nice (though the last two feel very situational) but is it little more then an oh snap button? How often should I be using this and should I be following it by deactivating Darkside to rebuild mp?