maggot said:
Funny but in every campaign I've played in, not one wizard has ever used that rule. They were too worried that having an empty slot meant less fire power.
I've known Wizards like that, but they inevitably were outgunned by whatever Sorcerer or Psion was in the group (and occasionally even the Clerics). You just can't cast as many spells as the Sorcerer can. It's counterintuitive, since the classic fantasy wizard was all about the firepower, but that's just not how the 3.5E system shakes out.
So they learned quickly; while a Sorcerer with a bagful of utility wands is arguably stronger, the Wizard gets this for free. Leave a few key slots open (no more than 1 per spell level), and your group will love it... assuming you bothered to learn those utility spells, that is.
The other big advantage of day-to-day prep is that certain spells (like
protection from elements) aren't really useful on the average day, but when you do need them, you tend to want several. If you know you're going up against a red dragon, for instance, you'll want each team member to have one set for fire. Under your system, that's not an option; if I allocate one third-level slot to that spell, on most days it'll be a wasted slot, but on days I need it it won't be enough.
And then there are the spells that are only useful on non-adventuring days. Divinations (especially
identify),
fabricate, that sort of thing. Would anyone take those if you were forced to use one slot for them each day? (Well,
fabricate, if you can get your DM to take a loose interpretation of "raw materials".) Or how about the various astral/ethereal travel spells, or
plane shift? Those aren't something you'd use every day.