(1) how much can one level up the Profession skill before the return on investment begins to diminish? I.e. What's typically the max to which a profession skill would be leveled?
(2) Is it feasible to pick up a new profession each level, role-play permitting? Are there advantages to this?
I.e. When in the big city, a curious fighter might enjoy spending his evenings going to the theatre, and through happenstance might find himself getting involved in the ticket scalping business. Once he's in with whatever semi-professional street guild organizes those scalping activities (in the casual time between adventures, mind you), it seems reasonable that he might be involved in other aspects of their business network, such as being a bookie, or at least keeping the bookie company as he works. i.e. "hey, you're a big guy. Why don't you hang out with me as I attend to some business?" This would function as an internship and apprenticeship, and a shrewd character might be able to pick up the tricks of the trade over time. And as this character becomes cozier with his shady friends, he may hang out with them during their day jobs, such as a tailor or an inn keeper, even helping out on occasion.
And so on. I think there are a variety of professions which wouldn't be too difficult to learn during the short, intermittent spurts between adventures. I.e. any profession in which "on the job training" lasts perhaps a session or two.
For this particular impromptu character, I also like the idea of him submitting play-reviews as sort of a freelance journalist. Or perhaps becoming a playwright or editor himself, as he uses his visits to the theatre to network and ask for advice.
Anyway, I think this is cool, but I wonder how useful it is.
The answers are heavily depenant on your GM's play style.
Unless your campaign calls for a lot of profession(sailor) checks, Profession is a f%**ing terrible skill. Talking about "diminishing returns" doesn't make sense, because there are zero returns in the first place.
Absent Ultime Campaign, of course. But call me Sally if the road to mediocre riches and minor cicumstance bonuses isn't "max out one skill and oh, you can use an actual skill instead of Profession".