Scene stealing NPC's

[KorvinStarmast]

I’ve had plenty of Mary Sue NPCs that the PCs loved, because they stole a scene, had their cool character moment, then exited stage left. Plus, it’s a lot easier to keep an NPC cool and mysterious when they’re not constantly participating in the story.

Those can be great NPCs.

An NPC from the very first encounter of the game which only had the word 'cowardly' beside it as a description is now a fully functioning member of the party which the only influence I had as the DM was allowing it to happen as events unfolded.

That's something I've seen with some frequency, going back to the dwarf we met in the 3d level of the dungeon (OD&D days), who was the last member of a band of dwarves our party met whom we - wait for it, all ye murder hoboes - did not attack upon encountering them in the dungeon. We parleyed with them and decided to travel together for mutual protection.
And then we got into this huge battle with a large group, many dozens, of goblins and hobgoblins and an ogre - the dwarves were our allies in the battle. All said and done, Gar, whom we named

Spoiler: Gar Goblinbane

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when anyone rolled for his attacks, the DM had the players do this since he had a horde of goblins to take care of, it was uncanny how the die got hot.

) was alive once the battle was over (a few PCs and dwarves had died) and joined in with our party to head back to the surface and dispose of our loot... much of which we turned over to a wizard in repayment of the clone spells he'd cast on each of us "on consignment" ... at a hefty interest rate ... so we were broke again.

A few sessions later my brother (older) joined us and in the interest of 'just joining in' where we were the DM offered him Gar as a ready made character. Done deal. The NPC turned into a PC, and he was still alive when the campaign ended due to various people going off to college and such.

Okay, it's not always the case, but this isn't what the OP was talking about. Your examples aren't stealing the lime light.

If people will rotate being DM some of the motivation for a DMPC goes away, it seems. Well, that's my experience anyway.