The short answer: a bit of out-of-the-box ad-hoc discoverability for users who won’t need to reach out far nor have good Google-Fu.
Anil, you are very right, that the stability of the opam package-collection is a signal strength. The careful and comprehensive testing that occurs before a new package-version is released is a real joy to watch and benefit from. And you’re right, that there are ways to get the utility of opam for unvetted code, by pinning package-names to git URLs for example. But to your question, even though I didn’t propose this, I immediately thought of one good argument for it: ad-hoc discoverability.
It’s true, that people can just put up a public github repo and make an announcement (or not). With Google search, people can find that repo and try it out. But this is … well, this isn’t as “discoverable” as being able to publish a little blurb to a standard place, where (for instance)
opam search can find the package and let users know about it.
I think it’s important to carefully weigh “easy discoverability of unvetted packages” against “stability and dependability of the repository seen as a whole”. As someone who was an early Debian user, I used to laugh at my friends would run on
unstable or even
experimental when they made their machines unbootable. I typically ran
stable, and eventually start using
testing on one machine that I was willing to reinstall if necessary. That the opam repository is “stable”, is seen as stable by newbies is a really valuable quality.
Anyway, that’s my 2c.
ETA: I’m reminded of Ubuntu PPAs. They’re not as stable, and there’s a place to look 'em up (Launchpad) so you don’t have to search all over for them.