Does a rear derailleur guard provide effective protection?
I had a mate fall off with one attached it got bend and jammed the derailleur so bad we had to walk home, as the tools we carried were not good to get it off. I have seen many prangs where the derailleur took bigger hits with no problems. Most that I have seen (Department store ones) are too light and cheaply build to be anything other than cosmetic.   

It also depends on the bike - better bikes have a derailleur hanger that is sacrificial and bends, protecting the derailleur in many common prangs. Department store bikes often don't have a hanger, hence the derailleur takes the full impact.

I have though about it, but with years of Mountain biking, many falls (some big), only ever bend a hanger. They may work, but I asked - why do no mid/high end bikes them. Is it purely weight, do the want to sell more parts, or is it just not worth weight and money required to make a bike so strong it cannot be broken in a fall. 

I decided (for me) they a not worth while.....