The problem is that some reading systems might ignore the erorrs (missing elements) and appear to work. But, other software could fail. So if you make it such that it's not in spec but works now, you could later on have to go in and fix it for some different/newer reading software. It's not worth it to remove what's needed.
But one thing you can do is use FlightCrew to verify the ePub. It will tell you what is missing that you need.
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