Whilst mending a broken spoke by the roadside the other day (it was early enough into the ride that I didn't want to risk waiting until the end to fix it - and possibly damaging more spokes) the wind kindly blew my bike over and bent my rear derailleur even more than it already was and made all my gear changes skip. I didn't fix that at the roadside as it wasn't until I was cycling along I realised how bad it was.
It isn't actually the rear derailleur that gets bent, rather the hanger and all the articles I read mention removing the rear wheel and then the derailleur so you can straighten out the hanger. But unless you have the correct tool for aligning the hanger (you won't) this seems like a tremendous waste of time. It's easier to use a pair of grips around the body of the derailleur and the hanger and bend it out with it still attached - this way it's much easier to judge when the derailleur is vertical and thus you've bent the hanger out enough. With the derailleur detached it's much harder to judge how far to bend the hanger out (it won't even necessarily look wrong on it's own) and you risk reassembling everything only to find out it's still bent the wrong way!