Do we need origins?

Back last year, I suggested on another blog (don’t have the link since I switched laptops) that the reason for adding origins to so many characters was to make them easier for a mainstream audience to accept. Hence kicking off The Shadow with an origin story, or the original Conan the Barbarian or Solomon Kane (which was the film at issue in the blogpost).
In hindsight, I think I was wrong (even though crops did not wither in the fields and the sun continued to shine). Even mainstream audiences can do without them just fine.
Just consider, for example, ABC’s sixties rendition of Batman. All we got for an origin was Bruce Wayne’s passing reference in the first episode to his parents’ death. And judging by the show’s success, that’s all anyone needed.
Likewise, I’ve never seen a Batman cartoon (and since the 1990s, they’ve been drawing in lots of people who aren’t coming from a comics background) that started with the origin.
Even so, sometimes an origin is a good thing, a dramatic story with great characters. At other times, (as this Mighty God King post puts it), it’s “here’s my cast and I promise exciting fun times starting with next week.”
I blogged a few weeks back about the impulse to squeeze everything into the origin, even when it doesn’t fit. Now, thinking more generally, I’m wondering what makes a good origin (which is separate from what makes a good first story).
•Not too much set-up. If it’s the first story in the series, it definitely has to stand on its own, as opposed to just setting for “fun times starting with next week.” And even a good story suffers if there’s too much setting up (as I argumed in my previous post).
If it’s a flashback of some sort, it still has to stand on its own. Take Batman’s first-printed origin: It’s a short, two-page story that condenses everything down to the dramatic highlights (later versions could take the time to go into detail).
•A turning point. Kal-El’s planet blows up. Bruce Wayne’s parents die. Peter Parker turns from zero to hero, only to then pay a price for becoming a jerk. A wealthy black scientist realizes he was wrong to think racism and injustice are things of the past when a police bullet cripples him (in the TV movie M.A.N.T.I.S.——which never comes right out with the origin but leaves us to infer it). Sandman Slim gets out of hell and returns to Earth.
•Enough set-up for the future. Either it launches us on a story arc which will eventually wrap up, or it sets a premise for more-of-the-same (as I’ve noted before, I’m fine with both).
If you can get all that, I still don’t think origins are mandatory. Starting in media res, as if the series were already ongoing (as I did in Where Angels Fear to Lunch, though I didn’t think about it that way at the time) can work too and saves you all the set-up. Brain From Outer Space started much the same way, though the Applied Science stories turned out to be an origin of sorts.
After all, if everything required an origin, nobody would ever pick up a series book if they didn’t start with the first one.