That button was removed as part of the quiz navigation changes in Moodle 2.0, which was MDL-15452 and sub-tasks.
I don't remember exactly what discussion there was about that (in 2008). The key discussions to look for are all the ones before that about students getting confused by the proliferation of buttons at the end of each page of the quiz, and accidentally clicking the wrong one.
Then, in Moodle 2.1, the new question engine (MDL-20636) changed things in a way that makes it harder to bring bach the submit all button.
There is a good pedagogic reason for preferring an individual submit button on each question (ideally with one question per page). It is a good idea if students engage with one question at a time, trying to get it wring, and if wrong, engaging with the feedback and then having another try. Then they move on to the next question/idea. There are contexts in which what you want is better, but I think they are less common.
But, to answer your questions:
a) Yes, you could bring this back, if you are preapared to modify the code in mod/quiz/renderer.php to display the button (if the quiz is using an applicable question behaviour), and in mod/quiz/processattempt.php to to handle when the button is clicked (which requires a bit of understanding of the internals of the question engine).
b) See the issue numbers given above.
c) There isn't a feature request for re-instating this button. That you are the first person to ask for it since Moodle 2.0 was release in November 2010 should give you pause for thought before creating such an issue. Basically there is no point creating such an issue until you have a significan number of people in this forum arguing that it is a good idea. I think it would be a bad idea, and so would be likely to close such an issue as Won't-fix unless I have already seem several people arguing inteligently for it.