As an alumna who knows the behind the scenes workings of recruitment here is another reason Panhellenic asks for legacy status- the chapters want it. Plain and simple.
At many Universities, PNMs go through recruitment, even legacies, without a recommendation form. Unless the woman tells us, no one will ever know that she is a legacy. Don't forget that not everyone has moms and grandmothers who talk about their sorority affiliation and who would naturally fill out a recommendation form and/or advise the PNM to talk about this during recuitment.
Most chapters are required to report on how many legacies went thorugh during recuitment and how many joined their chapter. If panhellenic didn't ask for this information, they would be lynched by the chapter alumnae.
Finally - having chapters that cut a woman simply because she is a legacy of another chapter is, in my humble opinion, a campus culture. I would argue that changing the culture is going to do more good than eliminating a single piece of information. But then again, I'm not from the south!
And yes, I think legacy status will be very important with the new release figures, particularly with strong chapters, as most legacies are given at least one invitation to a second round as a courtesy.