If Nagios was a lightweight monitoring solution I would agree with you. Nagios however is a large monolithic system that is hard to integrate with third party metrics solutions, inventory management, etc. Though something like inventory management isn't part of monitoring it is still part of your whole system and if your monitoring solution can't easily integrate with your inventory management solution it gets frustrating.

I also tend to think service discovery or at least a flexible API that allows other systems to create new checks is a huge part of monitoring. Nagios' configuration files are a nightmare to deal with especially in a configuration managed environment.