I obsessively back everything up, possibly counter-productively. As a result, I have a lot of old snapshot and vmdk files.
Snapshots are not comparable to backups! A snapshot may allow you to revert to a specific point in time, but if something goes wrong with one of the .vmdk files, your lost.
These .vmdk files are used as a chain, where each chain link is important.
but I'm wondering (a) whether there is a quicker way, and (b) do I really need all those vmdk files?
a) You can select multiple snapshots in the Snapshot Manager, and delete them in a single step
b) Whether the snapshots are necessary depends on your needs. As mentioned before, each of the files is important for the VM, so never delete the files directly from the file system!
André