This is something that has come up during a discussion about a freeze break (bug 671693). In short, the idea is to let the user continue their session after a shell crash by letting them restart the shell with extensions disabled or let them go into fallback mode to avoid losing their unsaved work.

Andre Klapper:
> This comment is not specific to the requested UI change:
> If you "have disabled all extensions as a precaution" for me, why is the
> only available option to "Log out" (implies that I am logged in
> already), instead of just allowing me to continue to log in (now without
> extensions) which is exactly what I wanted to do?

Jasper St. Pierre:
> As I said, that screenshot is faked. The fail whale is supposed to
> come up in the context of a GNOME crash, in which case logging you out
> and logging back in is the best solution.
>
> See: https://live.gnome.org/GnomeOS/Design/Whiteboards/ProblemReporting

Abderrahim Kitouni:
> Still I agree with Andre: logging the user out is the best way for them
> to lose their work. If gnome-shell crashes, which is AFAICT the most
> common reason for this to appear, then I'd rather have the option to
> restart the shell without extensions (or maybe start the fallback mode)
> rather than losing my work.

Matthias Clasen:
> But this screen is really for unrecoverable situations - it comes up
> after your shell already crashed twice in a row. At that point, we've
> already failed the user pretty badly, and the best we can do is start
> over with some dignity.