The problem with that setup is that you get alerts that will get lost in the noise. Systems like Nagios, I can set up dependencies, so that I won't get a flood of alerts when the network goes down, or set up maintenance windows so that when I am working on a system, I won't get spurious alerts. Also, self-hosted tools like nagios mean that I can hook it up to send an SMS or use a backup connection, or run a script that triggers an arduino-controlled bat signal to alert me. None of this is really possible with DMS