It's about faking answers. AFAIK, one of the features of services like OpenDNS is that they will not deliver you some answers, or they would even deliver you some fake answers - like an information page about blocked website, etc.
This is something that DNSSEC technology prevents from happening. If somebody modifies the DNSSEC-signed data on their way, the validator will detect it and discard bogus answers. The same happens if the upstream resolver does not support DNSSEC and strips DNSSEC signatures out of the DNS messages passing through.