I think that most people file I-140/I-485 concurrently and apply for employment authorization at the same time. At that point they are not tied to their company

I think that most people file I-140/I-485 concurrently and apply for employment authorization at the same time. At that point they are not tied to their company exclusively, and theoretically they could even leave, if they are confident that the company would not maliciously retract their application. The risk there, of course, is that as soon as EAD is used to justify employment, H-1B is no longer valid, and if the initial application is denied, the applicant is automatically out of status.