Well, granting all you say, there's still two problems. First, the 'Game not Simulation' argument, but that's relatively weak. So, throwing even that aside...
It's no more logical that someone without ranks in profession(cook) can make a tasty pastry, than it is to say that someone with ranks in Profession(Cook), can cook anything equally well. So you either have to go down the road of sub-specialization (e.g., Profession(French Pastry Chef)), which quickly gets ridiculous, or admit it's an abstraction of the ability to cook professionally.
For a real-world example, I present my wife. She bakes wonderful pies. I mean really, good, easily the equal of what you get in a bakery, and so on.
However, I wouldn't expect her to be able to quit her day job and take up pie-making to earn her living. It's a different skill. So, even without 'ranks' in profession(Bake Yummy Pies), (i.e. she can't earn a living doing it) she can still render a delectable treat when inspired to do so.