kailashr, I'm pretty sure that if and when I apply for citizenship, I'll be asked to fill out my GC approval date on a bunch of official forms and, even if CIS does not double check the info, I personally won't have the ba##s to knowingly fill out wrong info on those forms. So much for CIS's inadvertent effort to make it up to me for its long delay.
On the other hand, I would like to avoid the experience of being told that the card is invalid because of the wrong "PR since" date printed on it. Hence the concern. I do appreciate your thought though.