Key going up = Harder, if you have a hard time at that key level, pushing it above what you can complete won’t make life any easier.
Key going down = Faster, easier to clear.
Key complete = Gear, valor, progress.
If you want gear, you want to grind the level of key you can do consistently, quickly, and easily. Some groups stop at 19, some stop at 15, some stop at 9, some stop at 5. I personally find 9s to be the best to farm for gear right now until everyone is geared to the teeth and can push through 15s easier since Thundering is kind of a meh affix that will inconvenience you at the best of times and wipe you at the worst of times.
Just because people look and say, “Ooh, big ilvl, big IO” for invites doesn’t really change the system that there is zero reward for leaving a key and the only real reward you don’t get when a key isn’t timed is a number that goes towards achievements and the highest you can upgrade an item with valor which also won’t be an issue this early since people won’t have enough valor to be upgrading items to 415 anyways.
You might not get an IO boost from a bricked key, but you might get an ilvl boost. From all of my runs so far, I have been getting 1 piece of loot every 3 dungeons without fail, like clockwork even. MOST of the time, it was useful. So far, only four drops I’ve had were less than useful (Poorly itemized staff, poorly itemized cloak, inferior bracers to what I already had, off-hand that I didn’t need). With ilvl, been able to get into more runs without waiting that long, which managed to get my IO up.
Treadmill of efficiency, if I get a Ruby 10 and it bricks into a Shadowmoon 9, that’s a plus because that Shadowmoon 9 will become a 12 for somewhere else and HOPEFULLY it won’t be Ruby again.