Well, given that there's no way to predict what setting/culture any particular GM will run his game in, there's likewise no way to predict what professions actually fit his setting/culture. To paraphrase Mark Twain, the difference between the right professions for a setting and nearly right professions for a setting may seem insignificant, but it's the difference between lightning bugs and lightning.
Personally, I consider having just a few archetypal character professions to be the same as forcing every GM to use the same assumptions the game designer used. Game designers are just as human as everyone else, they can't foresee everything. What's the point of leaving a GM the ability to customize his setting if he can't customize professions to fit it?
As an example, at the advent of gunpowder in a culture, there is likely to be some version of a grenadier within a single generation. It's actually not all that big a variant from the standard fighter. And yet if the GM can't create a believable grenadier profession, he has a glaringly obvious hole in what the players can do that FITS with his setting. There will still be standard fighters for several generations after, but the inability to create/play grenadiers makes the believability of his setting just barely fail.
Granted, few settings should have every possible variation available (I'm tempted to say none, but just because I can't imagine it doesn't mean it can't exist). But the game designer's job is to provide the GM with the tools to create the setting he wants, not to decide for him what tools he's allowed to use. Limiting how much the professions can be customized makes no more sense to me than leaving cinnamon and vanilla out of a spice selection because if you put them on steak the result is disgusting.
