Thats my take on it too. If I have an NPC accompany the party its for comic relief, as a guide or some other specific function. They don't stay, (or last long) and I don't stat them out other than race, a personality trait or two and vague "class/profession". They never do more than 1d4 or 1d6 damage, but they are active in whatever encounters the PCs are in. Having them vanish into thin air until they are needed is just unbelievable and too convenient. These days I wouldnt run a fully statted DMNPC with the intention of them being a "party" member and havent in well over 20 years.I think the key part of successfully involving these NPCs is that they need to be short-run only, funny enough to outweigh being annoying, and they can never outshine the PCs except for one epic moment where they do something awesome (then hopefully die afterward).
Part of this is: it's not impossible for there to be a dm-controlled character who travel with the party and makes the whole game better for being there. It's just very easy to mess it up and cause more harm than good. So most people who've experienced it will tell you it's a bad idea or should be used very sparingly if at all.There seems to be several of these threads with the results split between those who do not like them since they feel that the DM is playing a super character from another game and dominates the others and the others that think they are making a balanced NPC that just tags along with the party and feeds info while being a minor help to the rest.
Adventuring NPCs in the party are pretty much standard operating procedure around here, if for no other reason than if a party realizes it has a gap in its lineup (e.g. no Thief, or no Wizard, or could use another front-liner) they'll often go out and recruit someone to fill said gap.
What this means is that if for some reason everyone wants to play a Cleric the party's not then stuck with being all-Cleric: the flexibility is there to augment the party. That said, the players/PCs know there's drawbacks to overdoing this, mostly in terms of having to divide treasure and xp over more people.
That's on you as their controller to play these NPC adventurers as real characters and maybe push back if the PCs are trying to sacrifice them in the front lines or force them into a nothing-but-support role.I do try to avoid it, but if the story calls for it I'll do it. My main issue is to avoid players abusing non-player characters as meat-shields and healbots.