I can't really comment on that as I haven't really worked for that many clients yet. I'm still in school, after all
I think however that the general idea is, you do what your client wants from you. If he tells you he just wants an idea, then even a pencil sketch is good enough. Photo cropping/kitbashing is all just tricks about speed. Conceptartist is more about being able to get your client's desire into an idea that he can see in some way. In a perfect world, sure, you're davinchi and picasso combined- you come out with an awesome idea and then make an awesome painting- but you can survive being a conceptartist even if you are not god-level at art. I think. I hope. :p