While your answer addresses the obvious, I don't think it really gets to the heart of the question.
Agentless
- Lightweight, WMI or SNMP based
- Basic system gathering
Agent Based
- Requires the installation of a lightweight agent
- Local database retains information if connectivity is lost
- Provides greater detailed data collections
- Required to monitor applications, processes, services
I am sure there are other benefits to both sides but all I can think of at the moment.