Consider the following scenario:
-A base solution, created with various recording that can be re-used by other solutions. Let's take an example, Login recording for example.
-A solution C, which has a project that already uses some reference recordings, like the Login Recording from the base solution.
When you create another project in solution C and want to re-use the already referenced Login Recording of the project that was there initially, for the new test plan for the newly created project this is not possible when you drag and try to drop the Login recording into the new test plan. TestComplete wouldn't allow it.
Now you have to add reference the same Login recording from the base solution again in your new project, whilst it is already referenced in the project that was there before. Hence you have now two times the same recording from the base solution referenced for different projects in your solution.
I would go for something like project suite referenced recordings, which are only referenced one time, and can be re-used by different projects/testplans in your solution without the need to add the same recording each time for every project in your solution.