Another area to consider is the privacy of the record...Even though he didn't take the PSSA, he's no doubt in the system. Where are those records kept? Used to be in a warehouse in Minnesota...Since PA only has one reason for opting out, any discrimination (not getting into a certain college due to lack of a PSSA score) would then be religious in nature...
Privacy and security issues related to the longitudinal database concern are real; however, in this case, I'm not sure that a 5th grader who did not take a state test would be a reason for a college to deny entrance.
I don't think a fifth grader has much to worry about, but if a passing PSSA score is a requirement for graduation (unless the district has an alternate assessment), then there could be some situations where a learner with a PSSA score could have an advantage over a learner without a PSSA score. I think the opt out is an excellent idea and hope it becomes a popular parental or family decision.
The Federal law establishing NCLB explicitly states that the scores are not to be used for anything other than determining a school AYP. They cannot be used for college admissions, graduation, or anything else. Some states have require passage of state tests for high school graduation, but in PA, at least, there is also a portfolio option for those who do not take the test. Colleges absolutely do not look at these tests. They are meaningless. In fact, some colleges are even moving away from the SATs and other standardized tests.