Yes, they're reusable. But even if they weren't, they're very inexpensive.
The wreaths won't blow off. I've used them for years and they've never blown off. You shove the wreath's wiring down on the hook and it's held in place by pressure from itself. I guess if a hurricane comes through then, yeah, it'll blow off, but regular winds won't bother it. The trick of course is to get the appropriately sized hook (not too big, not too small) so that the hook can really get a good grip on the wreath's wiring.