To reiterate: 5th in ice time per game among D-men on the Penguins for the 2009 cup, and missed the entire 2017 cup run with an injury.
I was going to post the same comment. In his defense, Letang was excellent in 2016. He would have been my pick for the Conn Smythe over Crosby. But in 2009, based on average ice time per game, he was the Pens #5 defenseman (#6 at even strength, #6 on the penalty kill - but #2 on the powerplay). And, like you said, he missed the 2017 playoffs entirely.
(For what it's worth, I think Letang was a HOF calibre player, albeit a lower-tier one, but he didn't have a HOF career. Had he not been injured in 2017, a second Stanley Cup as a #1 defenseman would have been significant. He was only a Norris finalist one time - very few defensemen have gone to the HOF with just a single season as a finalist. But he had a good chance of winning it in 2013 had he not been injured, and there were several seasons where he played well enough to be a finalist, but injuries prevented that - 2012, 2015, etc).