Oddly, the strongest push for dropping the current version of S.515 may not be arguments against it, but rather the fact that the "Coalition for Patent Fairness" doesn't back it. That's probably a show-stopper right there.
Further, the new version of S.515 is more show than substance, and really doesn't get the Patent Office to a better place. In its current embodiment, S.515 proposes to add new procedures to a US Patent Office that doesn't have the funds to perform the old procedures. The folks in Congress do not want to address fee diversion from the USPTO.
Of the position of the Coalition for Patent Fairness on S.515, note the text:
The Coalition for Patent Fairness, an ad-hoc interest group that represents Google, Apple, Intel, and Oracle, among other high-tech giants, has condemned a recently unveiled patent reform bill. In the opposing corner are interest groups representing biotech firms, pharmaceutical companies, and smaller tech companies.
from Tech, Pharma Clash on Patent Reform