"but my question is how much does it help?"
How would you like us to quantify that?
"but I think a lot of people consider doing it so they have a shot at the big time (top 10 say) when there initial profile would exclude them. Under these circumstances, I don't think the Masters is going to do that much for their application. What d'ya say?"
I say that you may be mistaken. Performing well in a reputable MA program, developing good relationships with professors, and demonstrating a high level of research ability will do a lot for an application. Why would one think otherwise?