I'd say that heighten spell is primarily useful for sorcerors. As a wizard, you can afford to learn higher level versions of the same effects and prepare them if you need a higher DC. A sorceror, on the other hand, may only be able to know one or two spells per effect. So if the sorceror decides that he only can afford to know Charm Person but not Charm Monster or Dominate Person or Dominate Monster, when he really wants to charm one of the iron willed guards to the duke's treasury, he'll need to Heighten the spell to get a decent DC.