It probably would devalue some things, but not many are coming to mind. Also, I should clarify that by "merging all the combo staves" I meant require Lava, Steam, Mud, Dust, Mist, and Smoke Staves to make it, probably with some new additional item. The Combo Staves wouldn't be devalued since they are required to make it. The Tome of Fire would lose a bit of utility since it can be used with Combo Staves to give 3 of 4 Runes, but its buff to fire spells keeps it more than relevant. And in most situations, a Dust, Mist, or Mud Staff with the Tome of Fire would cover all the spells you need anyway.