I've got some good ideas in my head for a GUI frontend for managing LibAlts data, but it will likely take some time as I'm studying Ace3 tutorials and looking at other addons and such to figure out how to implement things.
I'd like to use the BlizOptions addon config GUI, but that would prevent the user from viewing the social panel to look at notes, ranks, etc. while interacting with my addon :(
It makes it easier to create addons that plug into each other for that data, yes. However, when it comes to figuring out which addon is actually storing the data and how to migrate it to an alternative, users aren't much better off. Basically, I feel that while it opens up opportunities for addon developers, it's also passing up an opportunity to free users from reliance on a particular chat addon or whatever.
As a user I also wouldn't want the alt data to be stored by my chat addon anyways given the choice, since they generally have horrible interfaces for defining lists of alts compared to, say, GuildGreet. It would be much better to have the data stored by an addon that is dedicated to providing a user-friendly UI for building and managing the alt data.
I feel that the idea as proposed would realistically force any addon using the library to maintain its own copy of the entire data set in order to avoid the inevitable issues resulting from users switching from one addon to another and all of a sudden losing their alt data because some random addon was the one maintaining the list. The same issue happens now when you rip out an addon that was providing a particular font or bar texture via SharedMedia, and it's damned annoying, so I don't know why it's being held up as something that can't be improved upon - especially when you consider the fact that we're talking about data and not static files in addon folders.
Edit: In response to Xinhuan's post: I think it's a good idea because it would allow people to create separate addons for creating and managing the data, as chat addons seem to generally have very minimal interfaces for that sort of thing. There are also other, non-chat-addon-related uses for such data; see GuildGreet as an example.
I'd like to echo Pasta's concerns. Most end-users would never think to run two chat addons (as an example) at once in order to migrate the data, and it may not even work because errors will likely result from running two full-featured chat addons at the same time.
I do really like the idea of providing a centralized registry for alt data though.