I will never get why anyone wants a metal case that has so many disadvantage. You claim you want to hold it with one hand but HTC One (m8) metal phone is slippery and even Android Central makes jokes about how hard it is for Phil not to drop it a few times every week.
Not all metal phones are slippery. The M7 certainly wasn't. The iPhones aren't. It was just the finish HTC used on the gunmetal grey M8 model that had a smoother feel to it.
To me, metal builds that are well done like the M8 feel great in the hand, and many people also think so. The M8 has been continuously praised for it's build and feel, in no small part to the metal build.
So far no one has every made a metal case that allows wireless charging.
I personally don't care for that. Certainly not for 9" tablet. Charge times are already slow with the wall charger plugged into a device, however charging is so ridiculously slow on a wireless charger and you have to mess around making sure you've got the inductor correctly placed on the wireless charging pad....ugh.
Just give me the micro USB cord to plug it in and be done with it. I don't want to wait around for hours to charge my device, and if I do want to access or transfer files from my computer to my device, it's already plugged in and ready to go. I usually charge my phone in the morning after I wake up, let it charge for 45-60min then go to work. Wouldn't get much charge on a wireless charger unless I leave it on the pad on my computer desk overnight.
Metal case is easy to scratch and you must me very delicate with out a case that my wife's one two month old Samsung Tab S is already showing wear so much she stopped putting it in her huge purse and taking it to work. Metal also shows all scares, scratches and dents. If I get a Nexus 9 with a metal case it will be inside a TPC case as soon as it comes out of the box.
I prefer metal body because to me it feels much, much better than any plastic and it looks better as well. Feels and looks like a premium product when you're using them, rather than just a plastic gadget with a screen on it. Yes metal can scratch, but so can plastic, and plastic has a higher chance of cracking compared to metal. It's a subjective opinion, but I prefer metal and I don't use cases on my phones and wouldn't on the Nexus 9 if it were metal.
Apple started this stuff about Metal being Premium with it's marketing campaign to sell over priced products when metal is a worst material than other man made material that allow wireless charging and resisted wear much better than metal.
Again, not everyone cares about wireless charging. I'd trade wireless charging for a metal build any day of the week. Once wireless charging improves to the point I don't have to put the phone/tablet on that stupid pad, then I'll care about it. If I could have the device in my pocket or somewhere else on my desk not on the pad and it can charge wirelessly as fast as wall socket charging, then yeah sign me up. Otherwise to me, there's no difference putting it on those charging pads and plugging it in.
So there are positives and negatives to everything. Your viewpoint doesn't match everyone else's, just as mine doesn't.