Why does Unity still not support library reference?

Every IDE supports adding a references onto other projects or libraries. The Visual Studio supports the solution.
It's very convenient because I can split my project into several modules.
But why am I still forced to import each model, asset, script into each my projects? And after update of the script or asset to do reimport them.
I hope the Unity Technologies doesn't pass over this question.

 

Use Visual Studio Tools for Unity, then you can add your own libraries to the generated solution without it stomping on them all the time.

 

You can already do this... Just put the compiled assembly in your project.

http://docs.unity3d.com/Manual/UsingDLL.html

I program on the Mac, but I build the assemblys with Visual Studio, Monodevelop seems to have some sort of problem.

 

I know it. That's not what I want.
I even can't put assets in assembly.