In the Dragonlance campaign setting, spontaneous casters are users of ambient magic, the magic leftover from the creation of the world that exists within everything. Arcane ambient magic tends to exist within the elements, while divine ambient magic exists within the living creatures of the world. A sorcerer is an arcane ambient caster, while a mystic is a divine ambient caster.
Ambient casters gather the magic from the world around them, shaping it into spells directly. In the case of mystics, they require a faith-based framework of some kind to pull this off, reaching within to draw upon the connection they as living beings have with other living beings. Sorcerers don't require such a framework, but they are limited in that they only know a few techniques (i.e. spells) and while they can cast them over and over they have to work harder to alter them (metamagic).
This solution arose when trying to implement the SAGA styles of magic into the 3rd edition of the game. In SAGA, sorcerers and mystics can technically cast any spell they want on the fly, but in truth they were limited to a narrow selection of schools (divination, electromancy, pyromancy, etc). Although we've had people say that they would have preferred a spell-creation-on-the-fly system for sorcerers and mystics, it's my belief that the D&D sorcerer (and the mystic, which is in the Dragonlance Campaign Setting book and is essentially a divine sorcerer with a single clerical domain) is a perfectly valid alternative.
Note that bards and assassins are also ambient casters according to this system, so like sorcerers and mystics, they did not exist as playable character classes prior to the release of Chaos in the Age of Mortals (which allowed mortals to draw upon ambient magic).
Cheers,
Cam