Avouz said:
Because the good save progression for any class increases dramatically faster than spell save DCs. Thanks to the nerfing of Spell Focus and the like, practically the only way you have of greatly increasing Save DCs is via Heighten Spell. As I've learned time and time again as a DM, tossing a unaltered Ref save spell vs. a rogue (or any other high Ref class) is pointless.
Sure you could argue that I could throw a Fort save spell on the rogue and nail him that way, but suppose I'd like to get his fighter and mage buddies too. How many area-effect or multiple target Fort save spells are there? Horrid Wilting is one, but it's an 8th level spell. Not many games I'm in are that high level.
With Heighten, I can toss a 5th level fireball and have a +2 bonus to that Reflex DC. (If I combine it with Spell Focus and Imp Spell Focus, it's even better.)
Now I might have a chance of frying the rogue. And I don't have to be 15th level to do it.
Why don´t you use a Cone of Cold, if you have a 5th level slot? The same DC, and potentially more damage (if you are above 10th level)? You have to prepare and cast it as a 5th level spell anyway, so where is the difference (And it`s even the same school as Fireball, so this shouldn`t be a problem for you if you can cast fireball)?
Or, since you want to attack a rogue (and since you don`t want to hassle with the area of cone of cold) , maybe Dominate Person (hey, you didn`t take him out of the fight! He fights for you now) or Hold Monster might be a better choice? How about using a lower level spell and throw an Ice Storm (no (reflex) save at all, and since a high level rogue will take half level anyway, regardless of the saving throw DC, the damage would be the same as your heightened fireball.)
The idea of a heightened sanctuary or Invisibility vs Undead sounds better, but these are two very specific spells, and even then you could probably solve your problem with spells of that level.
I am not saying there are no situations in which it might useful, just there aren`t enough to waste a feat on it. (it`s a bit like toughness for a 10th level fighter - how often will you survive because you have 3 extra hp)
Spell Saving Throw DCs do, by the way, increase at the same rate as good saving throws, if you only count the highest spell level available (this doesn`t apply for half or 3/4 quarters like Rangers or Bards). Good saves just have an intial +2 benefit - on the other hand, especially fortitude saves will increase at a lower rate, since constitution is usually not a primary stat (unlike dexterity and wisdom), but the casting attribute always is.
Heighten spell doesn`t change this formula, since they only allow you to prepare at an higher level then the original spell, not a higher level than you can cast.
Only Arcana Unearthed provides a good use of heightened spells - but there, they mean something very different.
Mustrum Ridcully
. Now after a few characters, and mostly because now we start at higher level usually, I don't take it anymore, but I still would take sometimes at level 1.
