I agree its not desirable to have only 1 MWS server but if you were only using it for basic things (broker admin, even BPM) AND this MWS has direct network access to all the DEV, TEST and PROD machines it needed to talk to, then this would work
hope this helps a bit. Still, go one per environment 
#MWS-CAF-Task-Engine#webMethods-BPMS#webMethods