Hello,
I prefer to have this skill on every build at least at rank 1. The reason for that is, that you are invincible, which makes you last longer in PvP (even if it is just some wasted attacks from the enemy players etc.), which makes you survive some AoEs on astral allods later etc.
Especially combined with recovery it's very good. Try use recovery first and then dark veil. Then you don't have to mind the global cooldown.
While leveling that maybe doesn't make a lot of sense, but together with blood ties you at least can survive a little bit. Are you still leveling? If so do you run on support build? Because leveling with support build is far easier than it is on assault. At least I think so, 'cause you survive much longer, can debuff enemies, buff yourself + you get blood for free even out of combat the whole time.