Some spells might also have greater (for at-will spells) or lesser (for daily spells) effects that a character who knows the at-will spell or has prepared the daily spell could use in a fight and regain after taking a short rest.
Take this further, based on the idea of Reserve feats from
Complete Mage.
As a class feature or a feat or what-have-you, you know some spells. You can cast these spells as
cantrips, at-will. You have some pool of vitality that refreshes slowly outside of combat, that you can spend to cast these spells as
sorceries. And then, you can prepare these spells in your spell slots so that you can cast them as
invocations.
Here's where the Reserve feats come in, the neat part: if you prepare a spell as an invocation, the cantrip and sorcery versions of that spell are more powerful based on the highest level spell slot you have it prepared in. As you cast your invocations, your cantrips and sorceries slowly become less powerful.
None of this is an artificial distinction, a gamist artifact; it's all based on the character's exertions and the passage of time. If you spend all of your sorcery points in an encounter, you might not have all of them back by your next fight-- they come back a point at a time, over time, while you're not using them. If you cast all of your invocations, your cantrips and sorceries will be weaker for the rest of the day. You have to pace yourself and rely on your cantrips or else you'll find yourself at a serious disadvantage.
Of course, Fighters and Rogues wouldn't have spell slots and daily powers... but they should have better at-will abilities and a similar vitality pool with which to enhance them.