Maxperson
Morkus from Orkus
Adventuring NPCs and DMPCs are very different.Adventuring NPCs (nowadays called DMPCs as, I think, a largely derogatory term) are a near-constant in our games/parties anway, so nothing new there.
A DMPC is an NPC played by the DM as if it were one of his own characters and often used to overshadow the PCs and "come up with" ideas to succeed at things. It's a blight upon the game as it takes away a large portion of the game from the PCs and is the DM putting himself into a role that isn't his to play.
An adventuring NPCs is simply an NPC with class levels that is going along with the group. In my experience it's usually, but not always, a cleric or rogue. Clerics are sought by the players to fill in the healer gap, and rogues the skill monkey gap for dungeon adventures. The DM will have the NPCs act like real people, but he's not using them to get ahead in the adventure and usually will not take a leading role in the party unless asked to by the PCs. Even then if the NPC isn't the type to lead it will often be refused.