Your clients should have a better idea of success or failure of a site than
PR. There should be some specific conversion that determines success. If those conversions aren't increasing, then the site isn't a success, no matter how high the
PR.
I've seen no proof that
PR affects SERP in any way. I've seen results for pages I've done with lots of lower
PR ranked higher than a page that has a higher
PR.
I've seen no such statement from Google saying
PR is a measurable statement of SERP.
Well, if your instant feedback is a little green bar, and not results for your clients, I'm not sure what I can say to that.
If we do measure ourselves on the
PR, then I'm a rockstar because I built a site that went from basically no
PR to homepage of 6, deep
PR through the site of 4 or 5 for most pages, and all within 12 months.
However, my success with that site was measurable beyond
PR because we took them from 0 lead generation on their site to over 1200 leads a year worth a minimum of $3k if they closed each deal.