Message Server is used by the Management Layer.
Genesys document says:
Message Server provides centralized processing and storage of every application's maintenance events. Events are stored as log records in the Log Database where they are available for further centralized processing. Message Server also checks for log events configured to trigger alarms. If a match is detected, the alarm is sent
to Solution Control Server for immediate processing.
You can find more about it by reading 61fr_refml document enclosed with 61 Framework, or just ask here
To put it simply, Message Server does the following:
store all the messages from the applications into a database (a simplified version of a log that you can then see with SCI)
when error occurs, tell SCS to do something about it
used by strategy builder in outbound solution to collect real ime statistics for call flow monitoring.
With WFM, you would use it to probably do ... good question

I was under the impression that Genesys insists that we install management layer no matter what if using G6.1... I wonder if we can do without all of that...
Have you tried using WFM WITHOUT Message Server?
Talking about WFM, how did you find it in 6.1? We have used 5.1.6 and 6.0 and so far, the results of our evaluation were rather dismal. It seems like there was something there, but yet, the packaging and manmachine interface was really something that reminded me of my 8th grade science fair project
Vic