While a Bonemancer would already have chosen either the use of Bone Spear or Bone Spirit as his primary attack, having a 15% chance to cast Level 10 Bone Spear on striking can help dish out more damage, but only if the Necromancer is willing to wade into melee. Most Bonemancers eschew melee, staying at a safe distance to cast their Bone Spears or Bone Spirits, thus reducing the value of this mod.
However, a more melee-oriented Necromancer, one who uses Poison Attacks, might find this trigger ability helpful, adding magic damage on top of his powerful poison attacks.
+2 To Necromancer Skill Levels
Good mod for Bonemancers, who need high levels in Bone Spear or Bone Spirit as well as their Synergistic Skills to be competitive in Hell Difficulty.
+100-150 To Mana (varies)
Bonemancers will appreciate this huge bonus to Mana, as Bone Spear, and Bone Spirit especially, tend to use up a lot of Mana. Having some other gear on that offers Mana Regeneration will work brilliantly with this mod, as not only would the Bonemancer have more Mana from the Bone Runeword, but once used for his Bone spells, he can refill it quickly as well.
All Resistances +30
Resistances are very important in Diablo 2, and Bone offers +30% to All Resistances, which is a nice, solid amount.
The +2 to Necromancer Skill Level, huge bonus to Mana and the solid boost to All Resistances make Bone a good armor for Necromancers, and Bonemancers in particular. However, there is one thing that Bone lacks: the Faster Cast Rate mod. Bonemancers rely on casting their offensive spells quickly, and Faster Cast Rate is of utmost importance to them in this regards. The quicker they can cast their spells, the faster their enemies will die and the less they risk death. To some Diablo 2 players, the relatively high cost of the Bone Runeword and the fact that it does not offer Faster Cast Rate makes it unappealing, with the reasoning that there are other, better armors out there.