Derailleur guard on road bikes?
What I would be concerned about is that derailleur guard creates a lever for bending the rear triangle of the frame in a fall. A frame is much more expensive than a derailleur. Kids bikes with derailleur guards are generally heavy steel bikes and kids don't weigh that much.

Bikes come with replaceable dropouts for exactly this reason. It's a sacrifice part to avoid damaging the more expensive frame. Road bikes aren't really designed to be crashed multiple times. 

If you're really hard enough on a bike that this is an issue for you, I would seriously consider switching to an internally geared rear hub. The cheapest way to do that would likely be to buy a new bike. Plenty of nice commuter bikes out there now that will likely stand up to the abuse.