I think the clerk with GTA did the right thing.
My mother was rather on top of things when I was younger. When I was a young kid, she would play Wolfenstein 3D only when I wasn't in the room. When I was 6 or 7, she would play GoldenEye with my brother and cousin, but I could only watch.
When I was 11, I was able to play Perfect Dark, but only if my mother was in the next room. Perfect Dark is rated M, but it wasn't as bad as GTA III + at all, which she would have never let me play if it was out at the time.
Basically, she was on top of what I was doing in the games and she kept an eye on the content of the questionable games I played.
Not every parent is like that, of course. That's why the ESRB ratings exist, obviously. The problem is, even for the parents that do read the ESRB ratings or are involved more heavily in the games their children play in other ways, simply knowing that a game is violent doesn't tell the whole story. I mean, there's a big difference between Perfect Dark and GTA, or Halo and Postal, all rated M, and all likely with "Violence" in the rating description, among other things. Even with T rated games, there's a big difference (e.g. Smash Bros. Melee/Brawl and Twilight Princess vs. Arkham City and GoldenEye), though that was somewhat alleviated with the E10 rating.
I think it's easy for a parent to think that an M rated game might be OK for their child to play just by looking at the box. However, it causes a bit of a problem if a parent just sees, "okay, this game is violent" but doesn't know just how violent it is. Therefore, I think it's a good thing the clerk showed the video to the parents.