I live in an area with lots of debris, dodgy paving, and rain, and to make matters worse, I fall down sometimes (I know!). The main things that get damaged, of course, are the brake handles and the rear derailleur. The brakes aren't so bad since the cinch just needs to be scooted back into place, typically.
On the other hand, the rear derailleur is a bunch of fragile, lightweight dangly bits sticking further out than anything on the rear of the bike, and takes a special tool and time out to bend back into place.
The question is, is it worth buying a derailleur guard like this one for a road bike, or am I going to get noob status, ruin the frame instead, or weigh down the wonderfully light frame?
Given that a replacement rear derailleur (Tiagra) would run me 30-50USD and the guard would cost 5-10USD, it seems to be more effective to get a guard. On the other hand, this bicycle co-op raises a serious concern about the axle:
We have to admit that we are ambivalent about derailleur guards. While a derailleur guard is useful in theory, the fact that the wheel track nuts or QR secures it in place causes misgivings. A derailleur guard can't help but interrupt the interface between the frame and wheel nut. This means that the wheel nut will be less secure, and a less secure wheel is potentially more dangerous than a whacked rear derailleur. Having said that, derailleur guards are usually reserved for kids' bikes where the rider will probably not have the strength to loosen a wheel while riding.
(Don't tell me not to fall down so much; I'm a lost cause ;)