The main advantage of writing pre-made companions is, *drum roll....* being able to logically tie them to the whole and develop their idea! In a PnP session, it's pretty much impossible to do such a thing if you are improvising. The same way a huge musical idea can't be done while improvising and it needs to be thought up and written down. Beethoven can't improvise his 9th Symphony :p. If you don't do this then the companions become token and they aren't any better than adventurer's hall husks. They literally mean nothing and they can be substituted with *anything*.
I disagree very strongly. As I've said before, PnP and a SP RPG are fundamentally different.
PnP and co-op gameplay is all about interaction with real people. Everything you do is made with the other players in mind. It's a collaborate experience with the interaction between players (and consequently, their PCs) at its core.
SP RPGs though are (for many people at least) about the narrative experience at its core. A story-driven game is basically a narrative experience with a certain degree of interaction and choices of various kinds at your hand. Since it's a narrative experience well written characters can exist for their own good, just for the sake of offering a well done narrative experience. Well written companions don't necessarily have to be at the heart of the story to offer something to enjoy. To compare such companions to mindless henchmen is both wrong and pointless. The taks of a mindless henchman is to be a tool for gameplay. Such a henchman has no narrative value or purpose. A well written companion though is completely different for a SP narrative experience since they blend into the overall narrative, which is much, much more than just the main plot or the main quest path. Following that logic every(!) side quests that is not fully and meaningfully integrated into the main storyline was ultimately pointless, completely ignoring that there there is a lot of fun and value to be found besides the main path. Calling all of that pointless is sad because for some people it's not at all that way. It's just a testament of an ultimate black-and-white thinking that misses a lot of what makes SP actually fun in the first place for quite some people. I mean, if you play a SP (party) RPG like BG2 like a PnP session in MP you're probably doing something wrong because such a game is not really meant to be played that way (which is imo prettyclear given their basic game design and the way they create their quests and characters and so on and so forth). Their main value lies in the emotional impact that comes from the relationship between the PC and their companions, no matter if they are fully tied to the main narrative. They can co-exist and still have a big value no matter what. So no, it's not necessary to tie them to the whole, and it's no prerequisite either (although there is of course additional value in doing so, no question here) since it's not the basis of why they are so important for many people.
Important explanatory note:
I think a lot of this discussiom has actually to do with player psychology and how we all play games - and ultimately experience them, on both an emotional and logical layer. There is little doubt that people experience games differently (empathy and its various manifestations are said to have a prominent role in that) and that different experiences lead to different wishes, opinions and expectations. That includes a whole range of topics from the question how one roleplays ("I am the character" vs "I care about/guide the character") to the question whether one is more interested in the whole picture of things ("logical") vs the emotinal impact of emerging gameplay ("emotional") (which is basically at the core of the discussion whether "choice" or "consequence" is of bigger important for an RPG). So I guess there is no one "right" way to play from the start, since we all play games in a certain way that is based on how our subconsciousness works. So we already start from different grounds when talking about game design and our convictions about what games should look like - and sadly there is little common ground between many of these manifestations. So what I call very important for a SP RPG might turn out meaningless or unimportant for sb else and vice versa and nobody of us is really right or wrong in the end (although I can get pretty angry if people claim that something is utterly pointless without ever thinking about the possibility that it might be of big importance for sb else who plays the same games quite differently...).