I use a lot of NPCs in my games, and I spend a lot of time detailing them - even relatively minor NPCs with whom the adventurers may interact, get a generic stat block and a four or five line history and personality summary, while major NPCs get the full treatment, with more involved and detailed backstories than most of my own player characters.
However, I don't consider any of them "DM PCs" - I don't have a character in the game. Non-player characters who join the adventurers do so because it makes sense for them to be there, not to fill gaps in the party or to "let me play, too!" A guide, an employee, a loyal retainer, a friend or ally that the players have made through their characters (not someone I contrive to force on the party) would all be examples of the kinds of NPCs that tend to end up with the adventurers for a long period of time in the games I referee.