Absolutely. I don't mean to suggest that nobody should ever use cloud services.
But you should be aware that things like this happen, and that there are policies which let them happen.
Dropbox might not be interested in blocking pirated content at the moment, but this could change in the future. Their polices already say that it isn't allowed, so what would you do if all of your pirated movies just disappeared one day?
Simple: I'd stop paying for dropbox. And not only if they did this to me, but if I found out from a reliable source that they were doing this to anyone.
I figure that's a good enough incentive for them to not try to monitor their files.
But yeah, I pretty much agree with your basic point to not solely rely on cloud services. Always have a physical backup of stuff you actually want/need.
Dropbox might not be interested in blocking pirated content at the moment, but this could change in the future. Their polices already say that it isn't allowed, so what would you do if all of your pirated movies just disappeared one day?