Hi Simon,
It's always the same question: "why should I pay for a product that just makes file copies?".
Because most backup solutions do not just do file copies! Depending on the software you choose, there are many features you can't have with a simple vmdk copy script you can build yourself. For example, you can do incremental backups, restore a single file inside a VM, deduplicate your backup storage pool, etc.
So choosing between a home made script or a commercial product for a backup solution depends on your needs. But the more VM you have in production, the quickest you'll see that you definitely need a real backup tool.
Good luck.
Regards
Franck