Great question btw.

Here's my opinion on the subject. If you deal with only one client/customer, then it doesn't matter if you send all queries to the Incident bucket. But when you have more than one client, and queries come in from all sides, creating cases adds that extra layer of segregation. And this segregation could be beneficial for a variety of things - like data privacy or offering white-glove service. 

You could argue all of those benefits can also out of Incident Management too. But you might have to custom-build in at least half of those scenarios. Whereas in Case management you get most of those benefits configurable OOB.

Again, there's many layers to this discussion. But I hope you got the gist🙂