No, the difference is when a great defender disappears offensively he doesn't disappear. That's the point. Shumpert's "on" all the time in an aspect you can't get enough of and that doesn't depend on needing the ball.
I agree with you 100%.
However, with Ben, we've forced him to guard a lot of different players throughout every game. The most evident case is probably the Lakers game a while back.
I believe he started the game guarding Kobe who struggled. He then switched onto Lou sometime in the 1st because Rondo struggled staying in front. He shut Lou down. A short time later, he went back on Kobe, and Kobe started getting hot. This forced him, to switch onto one of Clarkson or Lou...they did nothing against him. I think he finished guarding against Russell who didn't do much. In the 2nd half, I think he guarded Lou. Russell started getting hot, so this forced Karl to put Ben on Russell. Russell did nothing against Ben. Ben was then taken out in the 4th in favor of Belinelli..
I think I only counted 3 or 4 times where his man actually scored on him. This is the same game where Russell went 11-16, Williams 7-13, Kobe 10-19, and 6-14.
In short, it's really hard to judge Ben's defensive impact when he's being jerked around throughout the entire game in order to guard different players. This has a lot to do with the fact that Rondo, Rudy, and Co. are terrible perimeter defenders.
I'm not saying Ben is shutting guys down day in and day out. But, what I am saying is that it's much harder to catch when he's doing a good defensive job. He's never had a game this season where he only had 1 defensive assignment for 75% of the game. This makes it much harder to see how he's fairing against his guy.
We can look at Korver from last year. He's never had to guard PGs. He was hardly ever forced to guard SFs. Why? His team had Teague-Carroll-Bazemore-Schroder who are all + defenders. He never needed to guard them. He solely guarded SGs and it was much easier for us to see his defensive impact on the team vs. the opposing SG. With Ben, that's not the case.