My experience is that non-disclosure benefits all students during the recruiting process.
At Michigan Law School, they had a policy that required all firms hoping to recruit on campus to interview all interested students (at least up to one full day of interviews). There were 2-3 firms that declined to interview on campus because of this, but hundreds of firms and offices bit the bullet and continued to recruit on campus.
Now, this wasn't a strict grade non-disclosure policy because firms did eventually receive the grades, but at least every single student had the opportunity to interview with the firms they were most interested in. I'm sure that some students that wouldn't have been able to "make the grade" landed jobs because they kicked ass in their interview.
I think the bottom line is whether the school has the wherewithal to maintain the policy of grade non-disclosure, or whether employers are able to impose their will and force the school to disclose grades. Firms will probably continue to visit the most elite schools no matter what. At lower ranked schools, firms might only be interested in the top 5% or something, and if forced to interview all comers, they might just decide it isn't worth it.