I, for one, disagree with allowing this stacking.
The examples that are being put up assume that the character is spreading the wealth among other feats and between the three saves. In the extreme of this example, a character could drop all those feats into one save --
Fortitude: Death from Massive Damage, Coup-de-grace, and most save or die spells are now moot. Poisons are now useless as well.
Reflex: A whole host of spells are now wasted, because the character cannot fail a Reflex save.
Will: A whole host of spells are now wasted, because the character cannot fail a Will save.
If you play with this part of the game engine (the save boosting feats), you will deal great potential harm to other areas by opening up whole new avenues of abuse.
Most of these difficulties don't even require all the feats to be spent in one save -- even two or three make a character FAR more resistant to even powerful spells, because a spellcaster can take Spell Focus and Greater Spell Focus, but can't go past that (so they have a maximum of +4 to a specific school of magic); as an example, a person takes Great Fortitude twice, and SF/GSF Necromancy is now completely countered -- take it three times, and even these specialists of specialists can't hurt the character.
Summary -- No. I think this is a bad idea, that will see nothing but abuse.