That's going to be a very long list, but of the top of my head:

1. Much easier repairs since all schematics are known in advance.

2. Price. A lot of the price of the modems comes from licensing fees as others have already noted.

3. Integration. Directly embedding the pcb into your project will be much, much easier.

4. Collaboration. Working together in projects that utilize 4G becomes easier too. Many use cases might get revealed without the original creators intending for them.

5. Security. "More eyeballs on a line of code make bugs run away" and all that jazz.

Only disadvantage might be reverse engineer people might get bored not having enough things to RE :-)