As said, Shred is a dps gain, assuming a bleed is up. The glyph is only useful if you pop zerk or TF and the boss turns or moves so that you are no longer behind it, potentially putting you in a position where you may energy cap (spirit kings and empress come to mind), since you were expecting to use energy to shred. So long as you know the fights, you should be able to expect when situations like this may happen and be ready to use a mangle, but reaction times vary. I personally dont use the glyph in favor of rebirth, since just about every situation where the shred glyph is helpful can be avoided with better play.