Well, you can do other people’s keys and still get rewards like loot and whatnot, but in order for your key to progress it’s better to do your own key overall. Because you can decide who you want or who you don’t want.
Nobody is forced to use their own key, but if you want to be in control of who you invite or whatever then yeah you should use your own key.
They should add in something that lvls ur personal key if u run someone else’s, like if I have a lvl 5 key and I’m running others keys that are higher then at the end of the run should get something like the spark fragments that if u combine them it lvls ur own key as well