Absolutely it is worth the time!
Combustion also covers stoichiometry, which will come in handy throughout a broader range of topics, and it is the fundamental principal behind most of the air pollution field.
Finish up all the problems, and then work more (In Schneiter's 101 questions book, for example) if you have time. When I studied for the exam, what I would do is work through an entire section (Air, water, etc.) of Lindeberg's reference manual, do the example problems in the text, then do the practice problems from the separate book after finishing each chapter, and then do the relevant practice problems out of Schneiter's 101 problems book when I had finished allt he chapters related to that particular topic. I think I spent about 2 weeks total on "air" using this method. (and of course I hit it all again later with the practice exams, and extra reading after the practice exams once I identified certain things like conversions to standard temperatures and dry cubic feet as weaknesses).
