First of all, how does the argument "It makes D&D unique and flavorful!" add any weight whatsoever to the "burden of proof"?
This is basically another way of saying, "I like it!"
But here's my reasons why the "status quo" should change:
1. At its base level, Vancian casting reduces the sum total choice for its use to two things: spells and slots. Its design is to create a resource/use/consumption mechanic that aids making "interesting game choices," but in many cases the only interesting choice that gets made is: Which spell do memorize, and what slot does it take (based on metamagic)?
Let's take something like Mage Armor.
In D&D 3.x:
-It lasts 1 hour per level
-It takes a standard action to cast--with zero chance of failure unless you get interrupted/damaged in the act of casting.
In essence, the sum total cost to use this spell and gain its benefits is a first level spell slot, and a standard action. And with a duration based in hours, circumstantially, all it takes is a character to say, "I cast mage armor," for the rest of however many encounters they face that day, it happens.
Now take Savage World's essential spell equivalent, "Armor."
-It takes an action to cast.
-It requires a skill check to cast successfully. If double 1's are rolled on the caster check, it fails automatically, you lose the spell points for casting it, and you potentially take damage.
-It lasts a grand total of three rounds.
-It takes 2 power points to cast.
-You can increase the duration one additional round for 1 additional power point spent.
Now from the player's perspective, the 3.x option is the far, far, far preferable alternative. You choose the spell, you cast Mage Armor, and unless your wizard is getting the snot beat out of him or another wizard tries to dispel it at the exact moment he casts it, he or she automatically gets the benefit.
But consider the tactical use case changes that the Savage Worlds version adds to essentially the identical spell:
1. It only lasts 3 rounds. Not 3 hours, 3 rounds. Meaning, when I the player choose to have my character cast it, I'd better have a darn good idea how I'm going to use it. The tactical usage scenarios get a lot more interesting when the player doesn't get to just press the "On" button for a near-permanent armor bonus.
2. There's no guarantee that it's going to work at all when I do cast it. And the less proficient I am at casting, the greater the probability. Compare that to D&D--doesn't matter if I'm the lowliest of lowly magic users with an 11 intelligence, that spell just works. The end.
3. I can choose to increase the duration at the cost of my remaining stores of magical power--but it's only 1 round per point spent. I could, of course, choose to simply let it lapse after 3 rounds and cast it again, but oh wait, there's chance that the 2nd time I cast it it doesn't even work.
4. If I critically succeed at my magic casting Trait check, (aka, get a raise), the spell actually works better than if I just have a normal success. Now I am incentivized to build my character to more effectively use magic not just to avoid failure, but because maximizing success actually means something.
5. If I fail the Trait test--i.e., roll snake eyes on both trait check dice, it backfires, and something not good happens (usually potential injury).
Based on this difference in use case, what does that do to me, the player actually wanting to play the caster?
1. It means I am wholly, totally invested in making my character the best damn magic-user I can, because I want my chance of failure to be as low as possible, and to increase my potential for critical success. "Using magic" suddenly takes on a whole new tactical and in-play flavor because, well, if I screw up, it's not just a "ho hum, lost that 1st level spell slot." If I screw up, I hurt myself. And if I critically succeed, I get a little more than I bargained for.
2. My "Mage Armor" spell is no longer a semi-permanent bonus. "I cast mage armor, the end," simply doesn't happen in Savage Worlds. And if a wizard uses two 1st-level slots in 3.x, it changes from a "spell" to effectively an "always on armor bonus." Now to some people that may be the actual point, I suppose. And yes, it's a choice by the player to use or not use those particular spell slot resources. But the Savage Worlds scenario takes a spell that by and large is taken for granted and turns it into something that requires more thought, planning, resources, and tactics from the player. And to me, that's a very, very good thing.
In 3.x, Mage Armor is a largely static, unchanging choice, that has no implication on future use throughout the entire course of the character's in-game life. It works exactly the same at first level, and exactly the same at 20th. "I want a +4 to my AC, so I memorize mage armor as one of my 1st level spells, because it's foolproof, it always works, and I can even cast it on a friend if I need to."
Why not just permanently give the magic-user the +4 armor bonus and be done with it? And that's certainly the logic 4e took in this instance by essentially leveling out defenses to a certain degree. The cost-to-effectiveness ratio for this particular spell is almost nil. At the cost of 1 spell slot, I get X. Always. Forever.
Of course, some will simply say, "Fine, then play Savage Worlds," which is a valid answer, I suppose, but the idea here was to point out that there can be significant changes to the overall feel and structure of a magic system that create different, interesting choices. And Vancian magic in its current iteration (I do realize that I could houserule it) does not have the same ability to model the same types of interesting character and tactical choices without some massive restructuring.
But another reason to not simply play Savage Worlds all the time is because I happen to like a lot of the 3.x rules structures. I like feats. I like the general feel of class progressions. I like the campaign settings and modules based on the 3.x rules, and don't always want to have to convert them on the fly to another system. As a GM, there are a lot of compelling things about the 3.x rules. They're supported. They're popular. The industry's best producer of modules and adventures actively supports it.
And I'd like it even more if Vancian casting wasn't the default.
