Most people don't read the terms of service, but they do read the marketing copy. If the marketing copy contradicts the terms of service, the contract should be based on the marketing copy. The terms of service should not be a license to make false or misleading marketing claims.
If the marketing copy said something like "if we screw up, you can sue us" while the TOS says "we can't be held liable if we screw up", then that might be relevant. However, I've never seen dropbox say anything like that in their advertising, so there doesn't seem to be any contradiction.
