Historical evidence suggests that people will create their own repos if it's easier (one of the reasons why tooling around QA needs to be so good) and that this does cause problems.

In particular, developer A packages a library into their repo because it's needed for their app. Developer B packages the same library and puts it into Extras.

User comes along and has both Extras and dev-A's repo enabled. There are three problem cases:

1. The two libraries have the same name, but different versions. Both apps require specific naming; but there's been no collaboration or awareness.
2. The two libraries have different names but potentially install into the same place. Again, no standards or collaboration on naming.

These aren't theoretical problems, unfortunately - we've seen them in the past.