The Marketing Cloud ID Service is handled by the Audience Manager (AAM) systems and some of the components of AAM (such as declared customer IDs) have been rolled into the MCID setup. Depending on what you are looking to do MCID is not necessarily required. It is, however, highly recommended and there are some setups that do require it (such as integrations with other tools). If you aren't planning on using any other tools and are looking at AAM as your only solution MCID may not be required. I recommend talking to your Account Manager about your specific setup and what you are looking to use the tool for in the future.
I am referring to Audience Manager in particular (as well as Target and Analytics- but that is secondary to me). My understanding is that MCVID will facilitate cross tool audience sharing (specifically segments to AAM). What will MCVID bring to table on the operational aspect of AAM? I am guessing not much? Again- just trying to better understand, thanks for supporting.