Spell components, such as Eye of Newt or Tongue of Dog are "Expendable Foci" if they have to be replaced for each casting . This can be a real hassle when Al's Arcane Emporium has just closed and you're being pursued by a ravening horde of demons who want to sell you a Really Excellent Set of Encyclopaedias, but it is traditionally appropriate for magic spells. The value of the limitation depends on how difficult replacement of the focus is.