actually, I do believe they ARE inserts for rainy days, but I was just like you, I thought it was a useless aesthetic part. You can remove them and make them into functional vents on dry days. Here is an ebay link. I contemplated buying this but I have more important things to waste money on before I do the exterior.
http://cgi.ebay.com/Mazda-RX7-86-91-...QQcmdZViewItem
I am seriously itching to hit "buy" but I need interior and random engine stuff first!!!
Notice in this pic:
http://i196.photobucket.com/albums/a.../h8000cf_3.jpg
You can see the difference in the CF between the vents and the cover that was attached to the underside.