We need more people writing their T&Cs in clear ways. There are many advantages in a clear T&C, not just for the consumer but also for you. You are stating upfront what relationship you want to have with a customer and obscure T&Cs means you have an obscure relationship.
If even you can't understand your T&Cs then they're worthless, you actually don't know if you're covered or not.
Just because Dropbox made a muckup, looking like they let a lawyer convince them that they needed that to cover their ass when they didn't, doesn't mean we should all run round hysterically paying for arcane language from a lawyer.
I just don't believe they needed to write that clause that way.
Imagine if it had read something like:
By submitting your stuff to the Services, you are instructing us to use your rights as a licence holder or the copyright holder to copy, distribute, prepare derivative works (such as translations or format conversions) of, perform, or publicly display that stuff to the extent reasonably necessary for the Service on your instructions. This is to enable us to technically administer, display, and operate the Services. You must ensure you have the rights you need to grant us that permission.
In the end this isn't a new scenario, do you think a printer asks for a worldwide license on the stuff he prints? And yet they're somehow pretending it is. They just want to say that you're instructing them to use the file in a particular way and that you have to ensure you have the rights to do that, a licence or the copyright.
IANAL but it seems pretty clear to me that they let themselves get talked into putting a clause in there that actually wasn't clear.