Backup currency to try for a mythic is one way to use glory keys, just keep in mind that mythics are ten times more likely from a gem key and 100 times more likely from a VIP key. Also keep in mind that glory keys are the best way to get rares on a new kingdom release - without a decent supply, you’ll need about 70% more total gold keys (lets estimate around 38k for this next one) to mythic the kingdom’s rare troop if you dont put in any glory keys. Having at least a decent supply of glory keys in the mix allows you to get away with having less gold keys total and still mythic every troop. Note that, again, if you can’t reasonably mythic everything by the time the event ends, there is no reason to do the full dump, as you only need a slight amount more by the next kingdom, and you can dump gold/glory keys for low rarity fodder at basically any time but have a very small window to get your epics and legendaries reliably ascended via event keys. We ran some numbers on new kingdoms here if you are further interested.
Since gem keys only drop ultra-rare and above, they are fairly useless when it comes to obtaining new kingdom troops, as you get these from event keys anyways - easily mythic in the case of ultra-rares. Also, gem keys actually don’t have the greatest drop ratio for Epics, which is usually the bottleneck for high rarity troops from a new kingdom… only estimated at dropping about 2.7 times as often from gem keys as glory keys based on Shimrra’s chest stats and supplemented by my own data. An entire lot of 500+ gem keys might only get you 1-2 epics and 1 legendary (and easily zero, if you are not lucky), which only saves you around 50 event keys in the long run.
My guild rarely gets 40k seals, but when we do, it is always planned and always on the week of a mythic release. When this happens, I dump the seals first, since I’m going to dump them regardless for guardians as it will be a long time to the next 40k, then do my normal 50 VIP > all gem keys (unless the mythic dropped already) > 50 more VIP (if the mythic seems like it is worth it and I have extra). I should note, though, this doesn’t increase your mythic chances all that much, especially if you still have guardians in the mix, but if you get extremely lucky, you’ll have an extra buffer next month.
Event keys have been stated to have the same drop rate for mythics (if the kingdom has one) as gem keys. Taking just gem cost relative to gem keys and VIP keys into the equation, VIP chests are twice as efficient to spend gems on as gem chests are during a mythic exclusive period, and gem chests during an exclusive period are 1.5 times as efficient to spend gems on as VIP chests. Therefore, if you ever get into a situation where you are forced to spend gems to buy event keys, the average price tag to hunt a mythic will be three times as much as using them on VIP chests on any given exclusive week. However, this is still much better chances than getting them with VIP chests on a non exclusive week at this point, which is 11 times as much, even now. I haven’t had the opportunity to do this yet, but if I get to a kingdom event where I don’t already have every other single troop mythic and I’m missing the mythic in the kingdom, I might throw some event keys there just for the chance.
Note that this 3:1 cost ratio doesnt apply if you somehow have a massive supply of “extra” event keys, but you’d have to at least skip trying to early finish every other new kingdoms to build up a decent supply. Remember that if it comes down to it, you can also buy event keys with 180 glory as a backup, and that 1 event key is generally far more useful than 9 glory keys - in this instance, slightly more useful for the purposes of hunting mythics, since an event key is ten times as likely as a glory key to drop a mythic if there is a mythic in the kingdom. Won’t help you with Apocalypse troops, though.
I’m getting to the point where buying glory packs push me well past how many of that specific arcane are needed to trait every troop that currently exists, which is why I will almost never push a troop past mythic just for the stones. I have a rather large hoard of glory right now (140k+ and counting) that I plan on using on backup event keys. My reasoning is that it is far easier to farm two arcanes (400 glory on the best week) than it is to farm two event keys (360 glory on any week).