I actually agree with your manager.
Multiple projects means multiple assemblies, lots of copying around of assemblies, and generally slower compilation times.
If your only reason to have multiple projects is improved organization, then you are doing it wrong. It would be just as effective to use folders.
Some valid reasons for having different assemblies are:
- You have a plugin architecture
- You need to deploy assemblies separately
- You need to work in multiple languages
- You are creating libraries to be used in different places
