They would compost fine - no problem.

As to mulching with them...aside from the rather strange appearance ;) I would agree with you that wind might pose a problem as they are very light weight.

They might do better as mulch if you shred them coarsely and keep them moist.

Unsalted peanut shells make an excellant mulch, unfortunately most of use would not get enough to do more than suppliment the mulches we already have. Keeping in mind that to be effective mulches need to be 3 to 4 inches thick, at least, if you have enough to do that they may need to be shredded since dried peanut hulls tend to blow around in fairly light winds, other wise if they will be mixed into some other material (far better any way since there would then be a larger nutrient source) I'd not concerne myself overly about that.

I have used pistachio shells and they did not blow away. However, I had to do a lot of snacking over a long time before I had a significant amount.

I live about a mile from a peanut co-op. They have big piles of hulls out back for the taking. I was wondering if it was worth the taking?

Go for it. I'm envious. Unless they have been softened by the piling and exposure to the elements, I would be tempted to shred them. If they are piled and aged, you may find the older stuff at the bottom equivalent to aged leaf mold.

A low-tech possibility is also to put on heavy boots and stomp the peanut shells into smaller pieces. Kids might enjoy this and it would be a rare opportunity for them to be "destructive" with permission. Also, some people roll over debris with heavy vehicles to break it up.