When I've seen my buddies get hacked, I see the hacker farm instances with him, so this lock still has a place. Without it, the hackers spam the same dungeon repeatedly which hammers the instance server, causing problems for other players.
I put my faith in Blizz on this one, and just make sure I don't spam instances. The last time I got locked out, it was in Wrath and my own stupid fault for using ICC as a shortcut back to Dalaran, after doing a bunch of other short instances including VoA. Reflecting back on it, it was a dumb thing to do, and rather selfish to spawn a whole raid instance, just for my personal transportation needs.