My take on the Letang situation, combining this with what the HFPens insider said a little while ago:
1. The Penguins are really, really fed up with Letang because of how bad he has been this year defensively and with moving the puck, plus the rumor from the insider that Letang isn't listening to the coaches.
2. The Penguins won a cup last year without Letang, so Letang isn't a piece you can't win without. Letang simply isn't important enough to the Penguins anymore for them to deal with his horrid play and refusal to listen to coaches.
3. The Penguins at their best are playing defense by possessing the puck. Not having Letang doesn't hurt that much as long as you replace him with a good puck mover. You don't need a great defense if your defensive gameplan is "don't give up the puck". Best defense is a good offense and all of that. You could probably replace Letang with Mike Green and not notice anything if the team is playing the system it is designed to play.
4. The team desperately needs a shakeup, the team is incredibly stale and is vastly underperforming from where it should be at.
5. Letang will definitely have high value if he gets traded, just because #1 RHD are insanely valuable. His injury history hurts his value, it doesn't sink it. At worst, Letang is a top pair OFD capable of playing huge minutes, but he's an absolute defensive liability. That's what he has been so far this year. At his best, he's a Norris caliber 2-way defenseman. Teams that can afford him will line up to trade for him just because even a bad version of Letang is very rarely available, and that doesn't even touch on the upside Letang has.
So basically, I think you're getting Keith Yandle at worst if you trade for Letang today.