On a Front-End server the Monitoring Agent will not run unless the MSMQ agent is installed, but unless you plan to deploy a seperate Archiving/Monitoring role that agent is uneeded and can be left stopped.
Take a look at this previous discussion for more details:
http://social.microsoft.com/Forums/en-US/communicationsserversetup/thread/418720e6-4c61-46e2-81ac-21350c19e223/More details can be found here:
http://blog.tiensivu.com/aaron/archives/1867-RtcQmsAgent-fails-to-start-on-OCS-2007-R2-server-and-causes-KB-967831-April-2009-update-for-Front-End-Server-components-install-to-fail.html
Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS