I think about it probabilistically: random events happen, so the longer you go, the more likely a given mishap is to occur. But surely, bending the derailleur in is more likely than bending it out. I observe that they do get bent in through various means such as laying the bike down on its right side, being hit in a self-closing door, and having something collide into it. When I was a shop mechanic I saw it a lot, and the customers were unaware of what had hit their bikes. If I ever take a long tour, I will install one of those ugly things, and I'll probably install a dork disc as well. These things work, and I do not want a bent derailleur or hanger when I'm far from home.