The problem arises, though, when you conflate a continuity of character & context with a lack of stylistic variation. It sort of misses the point, I think. The two are not inextricably connected.
I must not be expressing myself very well, because I don't think that's quite what I meant. I'm not linking greater continuity of character and context with lack of stylistic variation, I'm linking (a) that continuity with tie-in fiction, and (b) a narrower style variation with tie-in fiction.
There's obviously more stylistic variation in tie-in fic with multiple authors than there is in a long single author series, I agree. But that's not the comparison I'm making. I'm not comparing multi-author to single author, I'm comapring multi-author to multi-author.Like, a long single-author series is clearly not going to vary much in style, and so that makes another valid comparison - part of the appeal of tie-ins is that several different authors get to interpret the same source material. So I'd argue that one of the appeals of tie in fiction is that you do get stylistic variation, while still keeping the continuity of character & context.
The style argument was simply another example of that argument. I'm not trying to trivialise tie-in fic by arguing that there's a lesser variation in style than in original fic, any more than I'd be trying to trivialise many romances by saying that they don't have as many gunfights as westerns. That would be comparing two different genres (romance and western) falsely, I think. I could do it, but it's an essentially meaningless exercise.
And that is what I thought the OP was doing when he complained about things like tie-in fic using the same bunch of characters to save the world over and over again. Of course tie-in fic is going to use the same bunch of characters! It's a facet of the genre in general... I don't think one can legitimately criticise something that intrinsic to the genre simply because it doesn't occur in other genres. It's like me saying, for example, that romance is worse than other genres because there's so much lovey-dovey stuff in it.
I think you'll generally get a better comparison comparing tie-in novel to tie-in novel than you will comparing tie-in novel to an original novel. I'm not, please note, arguing that either is better than the other. I'm simply arguing that they're different, and that you can't use exactly the same standards when comparing them, any more than you can use exactly the same standards when comparing a romance with a western.
Basically, my whole argument boils down to this: with apologies to the OP, I don't think he or she is judging tie-in fic by the best-fit standards, so his or her judgement is likely to be flawed.
Octavia, you preside over the assumption that a tie-in is limited by what it can accomplish because of its background. I maintain that a tie-in can accomplish what a solitary original non-series sf cannot: evolution of characters, settings, and events over large time-spans that makes the fiction wholly more believable and paradoxically allows writers to explore and use the wildly unbelievable with greater vigor.
I agree with you that tie-in has the advantage when it comes to that ongoing evolution. I am quite prepared to concede that. However, I don't think that has any inherent connection to believability. The most wholly believable SF novel I've ever read is Orwell's Nineteen Eighty-Four, for instance - Orwell managed that in a single novel. But believability is a digression - suffice to say that for me, believability is not related to that evolution.
My point is, again, that you can't compare that evolution with original fic because original fic (bar the endless series) simply doesn't have the same capacity for it. In general, an author simply can't put, in a single novel, the same evolution of character, setting, and event that an ongoing tie-in series can have. Therefore, it's not useful to compare them, to say that "There's more character evolution in the last ten books of tie-in X than there is in original-single-novel X."
I think that's fairly obvious, and something we can mostly agree on.
And I think, again, that it's a false comparison to say that tie-in is "bad" because it uses, as the OP says (for example), the same set of characters saving the world. It's false because to say that this is "bad" is comparing it - whether explicitly or no - to fiction where that is not the case. Like it or not, tie-in is a different ball-game than original fic, and to say tie-in is "bad" because that genre has a particular characteristic is like saying that westerns in general are "bad" because they have gunfighting in them. Western or tie-in may not be a person's favourite genre, but they're not fundamentally inferior simply because they exhibit characteristics typically associated with that genre.