I actually have the book Tao of Network Security Monitoring sitting on my desk though have actually read very little of it... its on the list.
I took a look at that beta for DPI that you linked to; however, it didn't look at all like what I would have expected. I guess when I am talking about packet inspection I am thinking something more like the product HERE. The DPI beta looked more like additional features that would be included as part of NTA.
When we turn our firewalls logging up I can see every accept and deny that takes place so the data is pretty granular. While I can certainly see value in a Packet Inspector, it seems like you are hitting a point of diminishing returns considering the cost to have something like that in place to capture everything you could potentially need. Maybe I need to spend some more time reading that book.