I'm not really concerned with oil "going bad" after sitting in a partially consumed bottle. What does concern me, is the possible mixing of multiple brands, batches, and weights that can happen after stuff sits around in the garage.
Sometimes containers get mixed up and the wrong fluid gets added to a partially consumed bottle of your motorcycle oil. When you find your baby is low on oil, you unknowingly add some motorcycle oil mixed with (insert any substance that is usually found in the garage).
Here's the biggest reason why I don't mix oils from different containers/ batches.
I used to have a 2001 Arctic Cat ZL 550 snowmobile. It ran perfect and had around 3,000 miles. It was oil injected and was fed a steady diet of Arctic Cat 2 cycle oil. One day, I found myself on the trail with about a pint of oil remaining in the reservoir. I scrounged a new, sealed quart of Spectro Sno 2 cycle oil from a gas station and topped off my oil tank. I made it less than 10 miles down the trail before my engine seized. Most lubricants are supposed to be compatible with one-another, however, strange chemical reactions can and do happen. Had I been able to find my normal Arctic Cat oil, I firmly believe my ZL 550 would still be running.
That blowup is my reasoning why I don't mix oil in any engine that I care about.