It's all about the File System
Chris, your statement about DXi is absolutely correct, this is an appliance designed for Target Based Inline Deduplication. However perhaps you are missing the most important fact.
The file system that sits below every DXi is StorNext. A file system that is absolutely designed for Primary Storage, in fact you could argue that only with StorNext are some high performance environments able to function.
Now I wonder who signed an agreement with Quantum a few weeks ago to sell Stornext?...would that be NetApp?