A former, pissed-off employee who still remembers all of your routers' IP addresses and SNMP communities can issue a SNMP request to shut down all network interfaces and disable your network to the outside world.
A former employee who tells someone else your SNMP communities...
A current employee who in a moment of laziness, inadvertently leaves your SNMP community in a public pastebin or Github Gist...
Even if you can only monitor things, instead of directly issuing commands, it's still information you're leaking.
Information leaks are still a class of vulnerability for a reason. It can give an attacker information on your network topology that he wouldn't usually have.
The less attack surface exposed, the better. Generally, if something is exposed to the Internet that has no (good) reason to be, it's a vulnerability.
A former employee who tells someone else your SNMP communities...
A current employee who in a moment of laziness, inadvertently leaves your SNMP community in a public pastebin or Github Gist...
So on and so forth.