Insight is popular among casters due to the relatively cheap runes necessary for its creation, along with the Meditation aura it provides. Instead of wielding it themselves, most players let their Act 2 mercenaries carry them.
Level 12-17 Meditation Aura When Equipped
+35% Faster Cast Rate
+200-260% Enhanced Damage (varies)
+9 To Minimum Damage
180-250% Bonus to Attack Rating
Adds 5-30 Fire Damage
+75 Poison Damage Over 5 Seconds
+1-6 To Critical Strike
+5 To All Attributes
+2 To Mana After Each Kill
23% Better Chance of Getting Magic Items
Prior to Patch 1.13c, a mercenary with Insight equipped would lose the Aura effect after dying. Taking off and re-equipping the weapon that gives the Meditation aura would bring it back. This issue has been fixed in the latest version of the game.