Earlier quoted context omitted.
You will have lots of dead code in your build. That dead code might have "dead dependencies" - transitive dependencies of its own, that it pulls in even though they are not actually used in the parts of the crate you care about. In the worst case, you can also have "undead code" - event handlers, hooks, background workers etc that the framework automatically registers and runs and that will do something at runtime, w…
Libraries like Guava and Commons don't have transitive dependencies - they are self contained except for other parts of the same library.
The same issue occurs whether you bundle all the code together or not, it's just that if you bundle it together you don't see what's happening and you can't use only part of it easily.