A PPL by itself probably won't help in the selection process. I think what most mean on here is that it is common that people with some flight time usually do well at Primary Flight training and therefore you have a better chance of finishing near the top of your class thus helping your chances.

I don't know if things have changed, but when I was going through the military didn't encourage flying on the side. I flew right up until I actually started Primary, then I couldn't justify the cost, or need, to continue to fly on the outside.

Baseops.net is a good resource for information, just be sure to do some reading on there before you jump in with both feet.

USMCFLYR

Edit: By selection process in my post I mean the airframe selection process after getting winged - not the selection process to be chosen for flight training.