Well both the main advantage and the disadvantages are: Vancian is quite potent and flexible on the bigger scheme of things. A vancian caster can rewrite most of his abilities everyday, in other words a flexible wizard can afford to become a different character in all but name each adventuring day.
Alot of people also like vancian casting because the stategy of choosing one's daily spells is something that some players enjoy. This is the argument in favor of vancian casting to which I am the most sympathetic.
That said, since we're getting into a discussion of the pros and cons of it, I just want to say I'm not really a huge fan of vancian casting in general. I made this thread to offer a compromise, since it seems that the developers of Next are unwilling to abandon vancian casting no matter what.
My problem with Vancian magic is that it mechanically imposes a narrative element in the setting instead of being able to mechanically model multiple narratives about the nature of magic.
Whatever the history or role of combat, skulduggery, or thievery plays in a campaign setting, the mechanics that D&D uses can generally model it.
A fighter knows how to use his weapons and armor. He doesn't "run out" of attacks at some point during the day or have to decide each morning whether he is going to be proficient with a battleaxe or a longsword today and be prohibited from weilding the other until tomorrow. Nor does a rogue run out of sneak attacks or get limited in the number of locks he can pick or have to choose each day whether he is going to be stealthy or acrobatic, but not both.
Of course, some limites have to put on magic, otherwise spellcasters would greatly overshadow other types of characters. But it would be nice to have the mechanics that govern spellcasting be abstract and flexible enough to allow those who write up settings not have to adopt the "you only have access to a subset of your spells each day and if you want to cast one more than once, you'll have to give up even more" as the only explanation.
I agree. One of the biggest complains I have about vancian casting is how much of a "dissasociated mechanic" it is. It just doesn't make sense to me, as in, if magic were real, I can't imagine it would work that way. I've seen many variations of magic users in a variety of literature, tv shows and movies, and I've never once seen anything like vancian casting in any of them (other than those which are based upon D&D, of course, and even they usually tend to ignore or at least don't emphasize the issue).
Alot of wizards use spellbooks, but they cast the spell right out of the book, they don't spend long periods of time "memorizing" or "preparing" the spell and then forget it once they cast it. That's ridiculous. Most magic users in fiction don't run out of spells per day, either. Usually, if there is any limit, it's because magic is mentally taxing and the caster becomes exhausted from pushing his or her limits too far, which makes far more sense than an arbitrary daily limit.
Back to the mechanical side of things, I think the 3.5 warlock did a great job of demonstrating that you can have magic that is at-will and yet doesn't break the game. I think you can balance spells in alot of other ways that make more sense than daily limits, and I think that daily limits in general are bad game design (see the 5 minute workday). Here's just a few examples of other ways of balancing spells:
* Casting Time. Let's take a spell like fireball, for example. It's a powerful artillery spell that hits several creatures at once. So how do you balance that against spells that do similar damage but are only single target? One way is to make fireball take longer to cast. If it took a full round (like 3.x summong spells) to cast a fireball, the caster would be vulnerable to disruption during that period, so choosing to use that spell would be a strategic choice and highly risky in many situations.
* Rituals. Some spells take a very long time to cast. Minutes, hours, or even longer. That helps prevent characters from using these spells frivolously. In the recent playtest article they were even talking about removing the gold cost from many rituals, which would effectively make them at-will.
* Limiting spells to one instance at a time. Many spells could be restricted so that you can only have one in effect at a time. For example, the wall of fire spell could state that you can only have one wall of fire in existence at a time. If you cast another, the previous one disappears.
* Temporary Immunity. Sells like Charm would be overpowered if you could just keep casting them over and over on the same target until he finally fails his saving throw. To prevent such abuse, targets could be granted temporary immunity to the spell if they make their save. For exmaple, the spell could say "if the target succeeds on its saving throw, you can't attempt to charm it again for 24 hours."
* Concentration. Spells that require concentration prevent you from doing anything else but moving, and they can be disrupted while you concentrate on them.
* A limit on maintaining spells. Casters could have a general limit that they can only maintain so many spells at a time. A good number would be their primary magic ability bonus, for example. This prevents players from stacking huge numbers of buff spells on themselves or others, or blanketing an area with dozens of crowd control effects.
* Fatigue/Exhaustion. Some very powerful spells might fatigue or exhaust the character. Maybe the wizard has to make a saving throw or become fatigued whenever he casts a particularly powerful spell like Time Stop, for example.
* Cooldowns. We've already seen this mechanic in breath weapons, and 4e had a recharge mechanic on monsters that is similar. Some spells, like the aforementioned Time Stop, could have a "cooldown" before you could cast it again, since it would be game breaking if the caster could cast it over and over again without limit.
* Special Components. This is another mechanic that has long been used in D&D. If a caster has to sacrifice an expensive item or resource to cast a spell, that obviously puts a strict limit on how often he can cast it.
* Preparation. Not preparation from a spellbook as we're used to thinking of it, but some spells could require some kind of special research about the target, strange alchemical mixtures, or other things that might not be expensive but still need to be prepared in advance. For example, the planar binding spells require you to prepare a magic circle that traps the creature to be summoned.
