Too bad I missed the survey. Oh well, here's my take anyway...
We've used adventuring-NPC party members since forever, they're just part of the game for us. They show up usually for one or more of the following reasons:
--- the party are missing a key component (usually healing or thieving IME) and recruit to fill it - this is by far the most common reason*
--- the NPC is along as a plot device e.g. a spy or turncoat or active observer
--- the NPC is provided by the module being run e.g. a rescued prisoner with adventuring skills (example of the last two combined: Teldroll Storis the Dwarf, in
Forgotten Temple of Tharizdun)
--- the NPC is met or found in the course of adventuring and is taken in, e.g. a would-be foe who switches sides.
Party NPCs are rolled exactly as if they were normal PCs, except if their stats are given as part of a module I'll almost always use those. In play they die about as often as the PCs do, earn xp and treasure shares just like the PCs, and so forth. I give them all character and personality; some end up loved, others hated, others are forgotten as soon as they leave the party. If an NPC looks like it's getting too powerful I'll try to find a reason for it to wander away, though there's been times in the past I've tried this with a popular NPC and the party actively talked it into staying.
* - for example: since covid I've been running my wife in a one-player game. She wants a party of about six adventurers but doesn't want to run six characters, so she runs two or three and the rest are NPCs. She does the rolling for all six; I roleplay the NPCs otherwise, or else she'd have no-one to talk to except herself when it came to discussing plans, etc. I make sure my NPCs don't always have the best answer or brightest idea (sometimes they do, sometimes they don't, and sometimes their ideas are just plain dangerous), and most of the time they do what they're told in any case.
In a more normal multi-player party I don't mind having at least one NPC, if for no other reason than if someone's PC dies the NPC is there to be taken over as a stop-gap so the player still has something to do.