Dr. Awkward said:
There has not been any suggestion on my part that this sort of thing should not come at cost.
I'm talking about resource cost. To use my fighter Power Attack, I need to purchase it as one of the powers I know (first cost; when leveling up my character), expend it as a per-encounter resource (second cost; this is merely an example). To use my stunt system Power Attack, I don't have to select it ahead of time (no cost while leveling), nor do I have to expend a per-encounter resource to use it. Sure, you could have it be situational modifiers and such, but this grows the core rules that everyone needs to know to play the game, rather than just the fighter rules (thus growing a smaller section of the rules for specific players).
I never said that you couldn't create some kind of magic-y stunt system.
I never said you did. I was saying that I don't care about a magic stunt system, because I find the idea of a freeform stunt system to replicate class powers to be a waste of time and paper, since it works against the fundamental design tenets of a class-based game: niche protection.
The thing is, a stunt system is usually expected to replicate the sorts of things that Conan does, not the sorts of things that Gandalf does.
The skill system can do that, given enough mechanical substance. We don't need a vague freeform system when we already have better tools to do the job.
Yet you don't make an argument about niche protection with regards to acrobatics? Shouldn't only rogues and rogue-like characters be allowed to do those things?
No, because class powers are explicitly what separates one class from another. Acrobatics doesn't allow you to completely invalidate all the class powers of the rogue class, but this proposed stunt system would do the same, since it's based on the idea of "I could realistically give this a shot," which is the basic premise that most martial class powers operate under.
If Acrobatics allowed you to sneak attack, or get bonus damage against a Bloodied target, or use the "I'm Batman" power, then I'd have a problem with it's relationship to niche protection, but they aren't even remotely the same.
It's essentially the same as allowing people to find traps without Trapfinding in 3.x.
Which, by the rules, wasn't allowed above a certain DC. If you notice, all my concerns were directed towards the use of a stunt system to replicate class powers without having to spend the normal resources associated with those powers, not the overlap provided by people taking the same skills.