Actually, currently there is partial support for this - and it is buggy and creates a risk of data lost. Please bear with me as the explanation will need to be long.
We use the Shift+Z feature for a kind of selective sync. We have the following structure in GDrive for client projects:
-- Client Folder
-----Client Project 1
-----Client Project 2
We also have a folder called "Current projects" in the root directory. When we need to work on a project, we just Shift+Z it to the "Current projects" folder, which automatically syncs it.
You will alsk "Why don't we use selective sync?" Because Selective sync would mean having to go down 2 extra folders (Projects folder and client folder) to access each project, and these are a lot of extra clicks.
You can't really recreate this with symlincs without creating one for each project - which is not the ideal workflow.
Now to the porblem - while adding a folder to "Current projects" does get InSync to sync it down correctly, removing it (clicking the X next to the folder name in the sidebar in Google Drive) does not remove the local files and preservs the link. The files do not appear int he folder in the web interface, but remain on the hard drive. If you delete the local copy, this deletes all Gdrive copies of the file or folder.