@andy : I get what your saying and if your super reliant on the barrier for survival I definitely would avoid the passive. It's not a requirement for any other skill, it's literally a last bit. But, lemmy throw this idea at you in relation to the KE.
Anytime you do damage with Spirit Blade you leech 30% of your damage delt as barriers. That passive consumes 50% of your 'current' barriers. So say your at 80% Barrier at the time of using Spirit Blade, it halves it to 40%, and you to 40% of your current shields and you do damage. Lets say you normally would do 2000 dmg, 40% more dmg ups you to 2800. 30% of that's +840 shields.
Hard to say how all that matchs up, what I saw of Barriers going up from Spirit Blade seemed to be around 20-25% of the barrier thing but that could easily of been due to not maximizing it. Maybe that passive actually does add straight damage, maybe it doesn't... not sure yet. Also we've seen 5k dmg before with spirit blade which is a pretty solid amount of barriers back.
Point is, at some point you'd start juggling low shields with a minor dmg bonus to Spirit Blade. Not saying that's Ideal, but if you weren't the main tank and your not taking the bulk of the dmg... it could potentially be a hell of a dmg boost to Barrier. Also vs Guard you'd do an absurd amount of dmg and leech back a balls ton of barrier in the process after part of its consumed from that passive.
-edit-
@andy again : "It's a passive, so getting it means you're using it nonstop." <--- your semanticsing your self into an argument. It's a passive, your right, BUYING it with a SKILL POINT means your USING IT. So don't BUY it so your not USING it. They're the same thing. LIke any skill, if it doesn't fit what you want, don't use it. And as he said (and I have said) the passive is specifically out of the way. It doesn't chain to anything, you have to go out of your way to specifically buy that one passive to use it so... don't buy it man, or decide to use your barrier for damage.