Why, because it makes dev-side easier? Or because it makes users’ life easier when they jump between Discourses? Something else?
First reason isn’t reason at all — as long workload is humane and a task is possible in this reality. Second one is good reason for MS or Apple, who doesn’t want struggle too much with customers, but otherwise — as an admin and content creator I want serve my users, not keep things similar than here, there or elsewhere 
So — the third option is the interesting one.