I'm not in sales (clearly), but the NAM product overview pages can answer this question:

http://www.cisco.com/en/US/products/ps5740/Products_Sub_Category_Home.html

For example, if you're looking to replace a software solution with a 6500-based NAM, here is the data sheet which lists the specific NAM benefits:

http://www.cisco.com/en/US/prod/collateral/modules/ps2706/data_sheet_c78-505255.html

From my perspective, having a dedicated piece of hardware always deployed on the network which allows me to quickly monitor my core traffic from remote -- and quickly change what I'm monitoring (be it VLAN, port, RSPAN, etc.) -- is a great advantage. If I see a problem in my lab, I don't have to worry about going do there to put a sniffer on a port. I can just setup a SPAN on one of my NAMs, sniff the traffic, look at my top talkers, etc. It's quite convenient.