Look at your picture again. If you forgot to fasten the strap, it could easily unwrap and hang down enough to catch the spokes of the wheel. The wheel will come to a dead stop in very short order since there is very little stretch in the strap. Your buckle can easily catch a spoke and has enough strength to stop the wheel. Hook a part of the buckle over one of the spokes and test it your self.

Bungee cords don't need to be adjustable because they are self adjusting. They stretch to fit. Of course you should choose ones that are close to the size that you need and not ones that are too long but that's not that big of a problem. As for the hooks, I don't use that kind nor would I suggest anyone doing so for a couple of reasons. First, the hooks can...and do...wear holes in bags and/or equipment. Second, like you said, they can slip and deform. They can also get caught in spokes but so can a strap buckle. Third, a tightly stretched bungee has a lot of energy. A hook style flings the hook pretty well if it happens to slip while you are putting it on or taking it off. I've been hit in the face and it hurts.

I use ball bungees which are shorter, don't a hook to rack the same way and they don't have a hook that get's flung up into the air at about eye level if one slips from your grasp. I use 4 of them for each corner of the rack and they are fastened in the middle over what I have on the rack like this



That doesn't make them dangerous but it doesn't make them as good at stabilizing the load

dwmckee said he got a couple of wraps around the wheel before it stopped it. acantor started this thread and said that he had enough time to stop before anything bad happened. Western Flyer had this to say about straps:

Bungees will stretch a lot before they break and/or stop the wheel. Straps don't stretch under that kind of load.

They use straps that have ratchets on them that allow them to put a lot of stress on the straps and lock the straps in place. They don't depend on friction buckles to hold the load.