I could lose any one of those without losing the bunch thanks to dropbox synching things.
If it's due to a hardware failure, sure.
If it's due to a bug as in this case, or an accidental deletion that went unnoticed for some reason, or corruption of a key file by the application that created it, or malware encrypting your entire filesystem until you pay a ransom, not so much.
Please don't consider a bunch of sync'd copies to be a complete backup solution, whether the mechanism is a RAID setup on your local machine, or an automatic sync offsite, or a Dropbox-style cloud service, or anything else. These tools are guarding against one specific and relatively common failure mode, which is useful, but they do not guard against a lot of other things that can and sometimes do go wrong.
>I have copies of my files on my work machine, my laptop, my wife's laptop, my gaming machine, and ofcourse dropbox's servers itself.
I could lose any one of those without losing the bunch thanks to dropbox synching things.
Read the story again. Jan had the same peace of mind you do, until a crash in the Dropbox client deleted ALL of his copies of the affected files. He only unsynced on one device, yet the Dropbox-stored copy, as well as copies he had on other devices, were gone forever, due to a client crash. Don't assume your files are safe just because they are synced to multiple devices; obviously that sync relationship can be disastrous when an unexpected bug rears its ugly head.
I have copies of my files on my work machine, my laptop, my wife's laptop, my gaming machine, and ofcourse dropbox's servers itself.
I could lose any one of those without losing the bunch thanks to dropbox synching things.
Sure it doesn't help in the case of deletion but its great for the more common case of "my machine suddenly won't boot".