I'm not really sure what you're asking - clearly having everyone in one class of service is not desirable since you don't have any distinction between levels of access to the system for admins vs. users and such. COS is also used for licensing purposes so if you have features (i.e. TTS) that have limited seats you need to use seperate COSes to divvy those up.
Beyond that I'm not sure what to say on this topic - if you have something more specific you're wondering about, we can try and help...