I'm a bit unclear as to the purpose of PLN Project Snapshots. If BIMproject Snapshots can capture model data as well as other BIM server data, and PLN Project Snapshots capture model data only, what is the purpose of PLN project snapshots? Why not just solely rely on BIMproject Snapshots?
Having PLN backups allows you to be more flexible. Some examples:
1. You wish to archive your project and keep a .pln backup for it. After 3 years, you decide that you need this project again (refurbishment, still not finished, any reason). You can open this .pln easily with your current ARCHICAD and migrate it to the latest version, while for the .BIMProject you would need the original ARCHICAD to migrate the project. Also, if the backup is coming from an older BIM Server (v20 and older) the .BIMProject can be used only on the exactly same BIM Server (same main build and same update build are requirements until ARCHICAD 20 6xxx and ARCHICAD 19 9xxx update).
2. Your BIM Server becomes corrupted / goes offline / is not accessible. You have a strict deadline. You can easily move on with the .pln backup you have and continue the work in solo.
It's up to you whether if you prefer to use .BIMProjects over .pln backups, but my suggestion would be to have at least 1 .pln backup as well. Sometimes it can be quite handy. : )