As I've said earlier: there should only be one undisputed authority on the ranking of law schools. If you have more than one authority on this subject, eventually rankings will devolve in their quality and credibility and be regarded the same way as polling results- sorely questionable and highly varied depending on their source.
I would argue that competition usually improves quality, not the reverse.
Also, while it may in fact lower credibility, this will only be the case if alternative rankings expose weaknesses in the system.
Everyone knows what really matters, I think. The main problem with USNews is that they don't do a good enough job measuring reputation and placement, which are the two things that really matter to most law students. An alternative ranking by, say, the American Lawyer, would probably be a helpful addition to the process.
(Such a ranking could do a more scientific survey of biglaw hiring partners, along with regional breakdowns of hiring preference. Actual placement could also be scrutinized more carefully, looking at what types of jobs are involved, etc.)