I forgot to clip mine down once but all that happened was they wobbled a bit at 70mph. I imagine the louvres stop too much of a pressure differential for them to fly up or fold back. I soon got out and clipped them down though!
Exactly my thoughts as
hereETA: Cain at Wolf Performance showed me these last weekend which may offer an even neater solution to bonnet security - click for source.

Apparently used extensively in the racing world so little doubt about their integrity.