Powers of spells is determined by level. A level 3 spell is a level 3 spell. There is no continuity when one of those level 3 spells for some reason can be obtained simply by taking a short break while others cannot.
Sorry, but I think you're wrong here. There has
never been an edition in which all spells of the same level have the same power. Compare
burning hands or
feather fall to
magic missile, or
gust of wind to
fireball.
All this talk of D&D lore
like it hasn't changed a million times already over the years is silly. Look at reserve feats in 3e; they basically turn a daily into an at-will. Encounter powers in 4e. Etc. If you don't like the idea, that's fine; nobody is going to force you to use them. But why are you arguing so strongly for stopping other people from playing the game the way they want to, too?
Plus, a level 3 spell takes no more rest to obtain than a level 9 spell. Just as it takes no less time for a level 20 wizard to regain a single level 3 spell he used than if he used his entire spell repertoire.
Oh, weren't we referring back to old D&D lore? Let's look at the 1e DMG on this:
1e DMG said:
Whenever one or more spells are to be regained, the minimum time required for complete rest (usually sleep) is that required for the highest spell to be recovered. Minimum rest periods are shown below:
SPELL LEVEL --- REST TIME
1-2 --- 4 hours
3-4 --- 6 hours
5-6 --- 8 hours
7-8 --- 9 hours
9 --- 12 hours
Oh dear, so not all spells take the same effort to recover after all- at least, if we're sticking to the old lore. Hmmm.
There's already levels of complexity in spells, they are called level 1 through 9.
Seriously, it really isn't that simple. Again, not all spells of a given level are equal. Why not give the weak ones something to make them worthwhile? And just shifting them down a level or two doesn't always make them worth preparing; I've still not seen anyone prep
gust of wind, even though it was lowered to 2nd level in 3e.
EDIT: Remember, functionally, an encounter power for a wizard is about the same as a wizard memorizing the same daily several times.