I don't understand. I've never been in an RPG I "resented". I can only very vaguely remember a game of GURPS that was lame, so I only played once . . . maybe the difference is that most of the people I've played D&D with I either taught the game or reminded them it existed, so we think alike? I dunno.
Anyhow . . . the only time I've introduced more powerful characters who actually adventured with the party, as opposed to a walk on conversational/Basil Exposition role, they were old PC's of other players. Actually, in both cases, these were PC's of players who are DM's, but of their own campaigns, not mine. I don't think there's anything wrong with bringing in a more powerful character developed in another campaign, but I have some pretty strict rules on incoming characters:
1) The character must have been homegrown, from 1st level. Every single level must always be earned -- if you want a new character, you start at 1st again, not the average of the party.
2) I check the stats and the gear thoroughly to make sure it looks "right". No test on GP value, just nothing completely broken.
3) The character has to have history in Greyhawk, where I run my games, or a plausible in game reason they got there from elsewhere.
One of the two more powerful characters was grown up from 1st level in a different campaign I run, and this was a way of exposing a newer party to the wider world. It fit the campaign arc, it suited the player (who was switching characters), and it didn't seem to disrupt anything.
The other was a player's first ever character -- with all the sentimental attachments that come with that -- who had retired near where the action was happening. The PC's actually needed a savior to prevent a TP+KoBK -- Total Party + Keep on the Borderlands Kill, resulting from me having much of the Caves of Chaos assault the Keep after one too many PC raids. Tossing an 8th level ranger in to help their 3rd level party and army of peasants didn't seem crazy, since they were sending out messangers, magically contacting birds, etc. for help. And the players didn't mind at all -- I've been told that insane, 180 round dozens of characters struggling across the Keep fight was the best fight EVAR.
I've never brought in a higher level PC that I, as DM, raised. But not because it's inherently "wrong", just because it's never seemed to fit the plot at hand. I do like to use my own -- and others -- retired characters as NPCs.
Another thought on NPCs versus "DMPCs" . . . often I let new players "guest star" by taking over an NPC or sometimes a DMPC party member . . . of the 8 characters in my email campaign, two were born as module NPCs . . . one is effectively a DMPC, the other is a real player character now. And a third character is a PC inherited by another player. And a fourth is a PC who's player was absent -- with me running the character -- for a while, and now back to being played as a PC again. In an email game, anyhow, it seems that characters are just characters, where voiced by Connery or Moore or the new guy.