DMPCS are a good 80% of the time recommended against. Having a character on hand to help steer the party where the DM wants them to go, or be plot critical for whatever story he has in mind, is usually seen as a no-go, even if said character isn't optimized to succeed in every single challenge with knowledge that the actual players didn't have access to, thus acting as wish-fullfilment.

However (except for everything past that last even), what if a PC filled this role? If a player's character was fascinating enough that the Dm used them for the impetus of more than a handful of the adventures that the party faces, is that the same kind of thing that might strike ire into a group?