Well Heighten Spell has a few other implications. In addition to the phenomenal ability to cast spells at higher DCs, this feat also vastly improves the flexibility of casters.
For example a wizard could heighten fireball into a 4th, 5th, 6th, 7th, 8th, 9th … level spell slot. So assuming that a wizard has 4 slots per circle, a 20th level wizard could cast fireball 28 times (4 times per circle starting at 3rd). Casters can move useful spells into higher slots, to get more uses out of them.
Although Heighten Spell is an extremely useful core rulebook feat from 3.5e, if there are already balance issues with casters, know that this feat will make them even better. So there is that to bear in mind too.