Fiberglass in the drip rail

[Cmillet]

I've read every post on "roof" "drip rail" "fiberglass" etc... And can't find anything about putting fiberglass in the drip rail. Can someone tell me why I should not? I am repairing the inner roof and can see daylight coming inn from the drip rail. (After I cut away some of the inner roof.) some of these are pin holes but some are just daylight between the sandwiched outer roof and windshield frame. Just thought I could use fiberglass to seal this off. Your thoughts???