I have been using this for more than 1 year and double clicking on definitions (right now 63.2MB) completely updates everything. Can be checked with windows update later. I do not think NIS are required to be downloaded (never did and never faced a MB sized windows update immediately after installing definition update).
Well, my explanation goes like this. Each time I performed a format and fresh Windows installation, I would install Microsoft Security Essentials and subsequently its latest definition updates (mpam-fex64.exe). When I check the "About Security Essentials", the entire line of "Network Inspection System Definition Version" does not exist. Installing Network Inspection System Definition Updates solves this problem. In my opinion, if you manually update Microsoft Security Essentials, either by using its Update button or Windows Updates, it automatically download and install both Microsoft Security Essentials definition updates and Network Inspection System Definition Updates. Such occurence does not happen if you use the manual update through Microsoft Security Essentials definition updates. It is my opinion, though. Correct me if I am wrong.