If you are a prosecutor, think of all of the time and energy it takes to understand an invention, get to know the prior art, and draft the patent application, claims, and responses to Office Actions. And how many months did you have to draft and file the patent application, or respond to that Office Action? And how many years trickled by after issuance before the patent was litigated? PGR is a way to quickly see how well you did on technical matters, because (as the bill is currently pending) it is not limited to issues of new prior art. (Of course, PGR will likely also test the issued patent in view of new prior art.) So, instead of years before you get some “feedback” on your work, it could happen in less than a year or two.
If you are a litigator, think of all of the time and energy it takes to understand the technology, get to know the prior art, and sift through the facts. If you are litigation counsel for the Patent Owner, then PGR provides at least an initial litmus test of the strength of the patent. It also provides the Patent Owner more confidence to allow you to move ahead aggressively, depending on how well the patent survives the PGR process.
As Deming would note, if there's something wrong with the process of examination, fix the process; don't add an inspection step. And certainly don't add a step for the benefit of third parties, like litigators. Currently available re-examinations can be used to challenge issued patents.
If one wants to see re-examination in action, see
US 7,910,531 overcomes KSR obviousness by asserting unpredictable outcome via allusion to Popular Science [!]
As to the deficiencies of post-grant oppositions:
Post-grant oppositions: another pothole in the road for patent reform?
Mike at TechDirt has it wrong on post-grant opposition