I've been pondering all this. The more I think about it, the more my opinion about this device - and B&N - improves. Let me explain.
The average person who is looking at the Glowlight 3 is someone who is already into the B&N ecosystem, or is about to enter an eBook ecosystem for the first time. That person would never notice issues like partitioning, or care. That person also might know little about the underlying Android OS. By being kept from that, so to speak, they won't inadvertently mess anything up. They will buy B&N content, and eventually learn about sideloading. For such a person, the Glowlight 3 is a fine e-reader. And such people are most e-reader buyers.
For people like us, B&N left a bonus that is hidden, but not too hidden. They could have made it a lot harder to access ADB, install apps, or root this device. They could have chosen a different OS. But they didn't. They might (quietly) assume (if they thought about it at all) that anyone who could figure out how to do such things will also know how to use that knowledge responsibly, and not totally screw up their device! (That's a big assumption, I know, but I'm staying positive!) But, as it is, if you know about ADB, you can install apps without rooting. You can install Calibre Companion and fill the B&N content partition to your heart's content. You can install your favorite reading app(s) to read those books. You can use Calibre and a USB connection to access the sideloading partition, if you wish. With rooting, you can remap the page turn buttons to work with your preferred reading app - and do other tweaks.
The biggest issues with this device may come from people who fall in between these two categories. Those who know more than people in the first category, but who don't know quite enough (yet) to feel comfortable doing most second category stuff. But, if you figure out what you do want from this device - within reason, given the hardware we have - it's not too hard to learn the skills to make that happen. I did. Some of you have, as well.
So, yes, we can all imagine ways that this device could have been improved to meet any of our tastes or standards. But, overall, this is a pretty good device - and it could easily have been a lot worse. It's easier to customize than a Kindle, and offers potentially more customization options than a Kobo, given the Android OS and rooting. Yeah, it could have been better - but it could have been a lot more restricted.
So, I'm going to actually (gasp!)
praise B&N for this e-reader! It may not be the 100% ideal device I might imagine, but it comes reasonably close, given that we're a niche group to begin with!
Such is my (somewhat) humble opinion....
YMMV!