IMHO, it's not a matter of network size. Either in big networks or in smaller ones, you need monitoring and in my opinion SNMP is the best tool for the job.
It has 5 main advantages in my opinion:
It's a Standard - it's the industry standard for monitoring. Most hw vendors support SNMP through their MIBs, so you can easily collect specific values and monitor them.
It's Cross-platform - works in *nix, Windows, Mac and every OS you can remember.
It's lightweight - ideally monitoring should be "agentless", but that would be in a perfect world and there's no such thing :) SNMP agents are very light and put no load in the servers.
It's extensible - you can extend SNMP agents with your own scripts very easily.
Two birds, one stone - you can use SNMP bot in monitoring tools (like Nagios) or in data collecting tools (like Cacti).