ersonally, I don’t generally use them. I didn’t even use the Event Torchic in X or Ash-Greninja in Moon (both of which are also non-native). I do make exceptions occasionally, though. I don’t mind evolving a native Pokemon into a non-native Pokemon, but that’s only a thing in HGSS with evolutions such as Misdreavus -> Mismagius. In my first playthrough of HeartGold, I used a Leafeon.
I did this in my last run of AlphaSapphire. Before I started the “second half” of the game (meaning, everything East of Mauville City), I brought in three non-native Pokemon from Sinnoh/Unova/Kalos and finished the rest of the game with them in addition to my starter and two Hoenn Pokemon. Was quite fun.