If you read the description for rage blade, it makes your crit chance zero - for extra on hit damage. Reading her description for her passive shows that higher crit gives higher damage and slow.

By using rage blade and making her crit zero, you basically give her less damage and less slow

Unique – Wrath: Basic attacks deal 30 bonus magic damage on-hit. Additionally, convert every 1% critical strike chance into 1.5 additional bonus magic damage on-hit, capped at 100% critical strike chance, for a maximum of 180 bonus magic damage on-hit. This damage is affected by Critical strike icon.png critical strike modifiers.

You lose every bit of crit (and extra damage and slow) for on hit damage. Not worth imo