Well yes, it's a small isolation failure, but a failure nonetheless. It may seem like a contrived example, but "HTTP_BODY" would be a perfectly reasonable choice of variable or macro name for someone building a tiny embedded web server, and then you have a user (the user in this case being a developer using your library) having to debug your code because its internal assumptions collide with the outside world in a way that's completely avoidable if you just put it in a separate compilation unit.