its very valuable. snmp reads and traps are often the primary means of communicating and monitoring with large amounts of networking equipment. its a standard, its free and its relatively stable.
1) make sure your server/equipment is patched up to the latest snmp server release.
2) turn off snmp writes, only enable reads. shall we say that again? do you really want someone running a script to reset your switches counter's to 0 every second so you lose all your MRTG stats and cant bill by bandwidth?
3) both cisco and ucd-snmpd allow you to setup an ip-access list on the server-side. use it to allow only your central snmp monitor machine to access it.
4) the community string isnt exactly a password - its not nearly that secure. just change it to something other than "public", its never encrypted, it can get sniffed on the way by, so dont use any of your favorite passwords. think of it more like an org specifier for a group of machines.
5) wait and pray for SNMPv3 with real encryption built in!
this is a good article showing snmp useage between different machine groups, its history, whats coming, etc.