The point is to STRESS test... That means they are attempting to push the server until it DIES so they know all of it's limits and how it performs under all loads... 
This time, the server crashed at 7,500, if they can get it working at 15,000 next time that means the server stability has improved... that means if the server is capable of running at 14,000+ without crashing, it will be quite stable and mostly lag free running at 7,500(what numbers will actually probably be under high-normal conditions after server launch).