2. They break.
Now, I’m not saying that the straps on your brand new fins are going to break the first time you dive them; in fact they’ll probably last for several years. The problem is that the thin rubber will dry rot over time and, eventually, will break. Experience has shown me that this typically happens on the dive boat when I’m putting the things on right before the dive and NOT when the fins are hanging in the garage where it’d be much easier for me to replace the strap. Go figure.
I’ve actually made field repairs to a broken fin strap using two zip ties, a cigarette lighter, and a rusty knife. It worked, but the whole ordeal was rather annoying and caused me to miss half of my dive.