agent:050 messages include a bunch of information that would be useful, but its worth detailing out what is there.
For agent:050 messages, you are looking for the following in a filter:
deviceVendor = ArcSight
deviceProduct = ArcSight
deviceEventClassId = agent:050
The message is generated every 5 minutes by default in accordance with the setting in the properties default file:
[ARCSIGHT HOME]/current/config/agent/agent.default.properties
agent.rawevent.logging.interval=30000
Assuming this is left as standard (which is highly recommended) then you will get around 288 agent:050 messages per day - 24 * 60 = 1440 minutes / 5 minutes for the raw event sampling data = 288 messages. You can also see the amount of data in the deviceCustomString4 field which is the number of bytes.
This is a status message though, so it will give you a break down of the amount of data that is processing through a SmartConnector and if the log source is down, then you will see a drop in the total number of bytes being sent, but you will still get the agent:050 messages. If your SmartConnector is down, yes, you will no longer see the agent:050 message and hence it will be suitable for a connector not being available, but the difficulty here is that this every 5 minutes and the worst case scenario is that you will have to wait for up to 5 minutes to have this identified.
So its a 1/2 and 1/2 answer - yes it might be useful, but its not great.
But the bigger question is why are you not using ArcMC for this purpose? Its a powerful platform for defining these things and you can do a lot with it. Also, have you considered the use of the Activate framework for this too? There is an Activate framework package for connector monitoring:
https://marketplace.saas.hpe.com/arcsight/content/activate-base
https://marketplace.saas.hpe.com/arcsight/content/activate-c-security-system-monitoring-base
https://marketplace.saas.hpe.com/arcsight/content/c-security-system-monitoring-connectors
And for ArcMC and what you can do with it for monitoring, I recommend taking a look here:
