I think the idea with Sanjo is that you can use death magic as a kingdom. Other heroes can benefit from the life shards, while Sanjo must do go without extra death power.
But some of Death's best spells don't even use or need death power to become awesome. Curse and oppression are both Death1 spells you'll use all game. Wither and shadowbolt is plenty useful with 0 deaths shards.
And Death 5, Sacrifice, requires no death shards and is so good it's almost a guaranteed win.