I'm playing an "illusionist" Sor8 in Piel's game. We've discussed the "spellcraft provides incontrovertible proof" idea, and there are some problems we can see with it. These problems come up especially when you're trying to get your friends to disbelieve something.
First off, identifying a spell via spellcraft essentially reflects the fact that you are able to recognize the words and gestures used to cast a particular spell. If you can recognize that something is an illusion via spellcraft, why shouldn't you recognize it by me shouting, "Hey guys, this is an illusion!" Or even sitting down around the campfire with your buds and saying, "OK, if you ever see me pointing up like this, and shouting 'brackenfrackenboom,' that's me casting Major Image." Or better still, developing a code phrase (like "BLUE FLAG!!!") that means, "I'm casting an illusion!" These all seem like good tactics to me.
But you run into problems when I cast Silent Image to mimic Obscuring Mist. All my friends know what I'm up to, so they can see right through it. All my enemies don't know, so they have to makes Will saves or suffer from concealment penalties. And I think that makes a 1st level illusion far too powerful.
It only gets worse when you start running into the Shadow Conjuration/Evocation spells.
We eventually agreed to say that the will save is what allows your brain to really understand that it's an illusion. It's like those magic eye posters. Someone can say, "Wow, it's a schooner," but that doesn't mean you can suddenly see it. In this case, we decided to say that these things (successful spellcraft, a pre-planned signal, etc.) should give you a bonus on your save, but not remove the need for a save entirely. Kershek's quote is very helpful, and it jibes with our reasoning.
Spider
Edit: This does raise another interesting question, though. How often can this be done? Just once/illusion? For example, could I cast a Shadow Conjuration: Sleet Storm in an area containing my friends, and each round shout out, "Blue flag! BLUE FLAG!!!" giving them another chance to disbelieve it with a +4 bonus? Could my other friends (who successfully disbelieved) also grant additional saves on their initiatives? What if my dumb friend still fails, and he tries to move...does his reflex save to avoid falling down get this bonus too?