Energy Drain

Because the purpose of Assassinate isn't to make you stand still for 12 seconds and then unleash massive damage. How stupid would it look if Shadowers spent 80% of boss battles standing in Dark Sight and waiting? The charge is like what Brawlers have with their Screw Punch, except instead of holding a button, you go into Dark Sight. It's a lead-in skill that allows you to put a lot of hurt on a target on approach. Same deal with that Windbreaker skill.

CSB's damage actually increases in DPM when you use it to charge-up (since you save on the delay). This is not they case with Assassinate, which you have to spam ASAP, because even if you wait 1 second in DS, your DPM will lower a lot... but ok, we're getting a bit off-topic here...