Well, this isn't a bad idea at all, but the thing is... nothing stops you from doing that already. People in general lack the ability to stick themselves to just a few skills or areas, but that's not a system problem, it's a player one. I keep myself within just a single area - crafting, and just the basic fighting (two skills: fighting and light armor, the law does force everyone to know some basic fighting). I admit I'm not totally specialized, after all my main crafting skill isn't implemented in-game and I've gotta do something, but still. Everyone right now has fighting, and magic, and crafting. And quests for everyone virtually possible so they can get x and y items (though on the quest part I'm not sure if I can blame much people. My character would do pretty much everything, unless quests that would require law breaking) or simply to do quest exploration (aka Power Questing?), instead of doing it to cultivate a specific faction.
So my point is: there's no need to implement this with a "job system". Everyone can and, hopefully in a close future, will stop being a jack of all trades. For now, I think it's good as it is. We're gamers and testers, remember? If most people do pretty much everything that means at least there's a lot of testing going on. And so eventually, while the system gets more solid, things will be implemented to force us to specialize.
P.S.: Re-reading the end of your post makes me think there's some real potential there, but it's mainly in "skill decay". You should, indeed, lose something from practicing something else, but the system itself shouldn't be with solid jobs, I quite like the idea of fluid free skills, make us all different.