The bonus damage dealt by Fray benefits from life steal.
Fray's on-hit damage does not apply to structures.
Wit's End is often most useful on auto-attack reliant, mixed damage dealers who deal both magic damage and physical damage, such as Shyvana or Kog'Maw, in order to deal more damage against champions with high armor by dealing additional magic damage.
Wit's End is also highly useful on frequent auto-attackers against champions with high magic damage, since it give its owner a good amount of magic resistance while not sacrificing an item slot solely for a defensive item.
Wit's End is viable on bruisers and tanks who rely on their auto-attacks to deal damage, such as Warwick and Shyvana, as it not only increases their damage output but also bolsters their magic resistance.
From level 1 to 9, Nashor's Tooth deals more on-hit damage than Wit's End. On level 10 they deal the same amount of damage. Afterwards, in order to deal the same amount of magic damage on-hit, a champion with Nashor's Tooth would need 150 − 325 (based on level) AP.
Previously, Wit's End gave 42 on-hit magic damage and attack speed. Considering the item's name, this was probably a reference to The Hitchhiker's Guide to the Galaxy, where 42 is the answer to "the Ultimate Question of Life, the Universe, and Everything".
New Unique Passive - At Wit's End: Basic attacks deal 15 − 80 (based on level)bonus magic damage on-hit. While below 50% health, heal for 33% of this effect's damage dealt, increased to 100% for melee champions.
Removed Unique Passive: Basic attacks (on-hit) grant 6 magic resistance and reduce the target's magic resistance by 6 for 5 seconds. Both effects stack up to 30 magic resistance. Ranged champions only steal 3 magic resistance per basic attack, up to the same cap.