We have been using the MC Visitor ID Service for some time now. When we first hooked it up, the advice given regarding setting customer ids was not to bother because they weren't used for anything at that time. I found this same information on some blogs written by Adobe folks and in talking with our internal Adobe contacts.
Is this still the case? It seems that if I associated, say, my customer'd account number with the the call that the service could use that to associate that same user across devices. None of the documentation I can find by searching specifically says how that value is or is not used, other than by detailing the setCustomerIds() function. I have tried asking our internal Adobe contacts the same question and, again, crickets.
So, in a nutshell: If we are not using Audience Manager, is there any benefit to be realized by including customer ids with the MC Visitor ID service?
Other than Audience Manager, I'm not aware of any additional products that explicitly and solely depend on the Marketing Cloud Visitor ID service yet. Analytics can take advantage of it right now, but it can also use legacy visitor ID's as well.
The device co-op announced at Summit this year will be using it when it's released, which will be hugely advantageous for those who already have the visitor id service already implemented. I don't have a timeline on when it will be released. And if you're running DTM, it's super easy to set up - basically just specify the tracking server used in Analytics and you're good to go.