I think it depends a lot on how it is used. In the case of sabre, that is somebody's job. They use that interface everyday so the user needs to be able to use t

I think it depends a lot on how it is used. In the case of sabre, that is somebody's job. They use that interface everyday so the user needs to be able to use the interface efficiently and is willing to learn the shortcuts. A sign up form that a given user will only use once in their lifetime is another matter. It needs to be simple lest the user abandons the process. Most users in that case are not willing to learn keyboard shortcuts, tabbing over optional fields probably makes more sense here.