There are many reasons for it:
- The game offers a new system for loading libraries that ensures that only the newest version of the library code is run. In the past an arbitrary number of old versions of the code embedded in each addon had to be executed before the newest one was found, increasing loading time and opening the way for bugs that are impossible to fix without having to replace all older versions of the libraries in each installed addon.
- The new library system allows libraries to access all features regular addons can use, like saved variables, XML UI controls, dependencies, multiple source files, etc.
- It takes less disk space and also slightly improves loading time, since the game doesn't have to check multiple copies of a library.
- Less "hidden features". Users can see which libraries an addon is using
- Abandoned addons will receive updates to the libraries they use, potentially making them live longer.
- Minion cannot update libraries contained within other addons, so installing them separately will ensure that they stay up to date
- Minion4 will be able to install dependencies automatically
- Authors spend less time updating bundled libs and as a result have more time for implementing new features and fixing bugs
And not many reasons against it:
- Users have to install dependencies "manually" a single time, until Minion4 is released