THAT system, complete with the plastic insert, is a manufacturer's affordable method of covering the joint-screws, and to give the whole thing a "finished" look.
I replaced all the screws with stainless steel screws (one by one), and replaced the insert (new insert stays on). I also removed the thin caulk on the upper edge of the aluminum strip. BUT, I used the best caulk/sealant that I know of,....3M Marine Adhesive/Sealant/Caulk #5200 Fast Cure. AND, before I installed each new SS screw, I applied a small amount of PEMEX Gasket Maker goop to the screw threads, and when I screwed them in, a little bit of goop squished out. Everything there is sealed permanently (my guess).
Oh, and yes, IMO it would not allow water to come in, even if the insert was not installed.