Written by Ty Schalter (who's fantastic), this is a great write-up on hybrid defenses and how the game has completely changed in the past few years, especially when it comes to traditional "roles" and positions.
I thought this quote in it from Jim Schwartz was really interesting, and is kind of a gist of the article itself.
We sort of got away (from WILL and SAM linebackers) because we saw so many shifting teams and teams get out of shifting real quick if they're moving four people and you got all these guys on defense going, are you ready yet? But if they're moving one guy and you're flipping four, they'll just do it 60 snaps a game.
That's why you start getting a little less compartmentalized with SAM and WILL, strong safety and free safety. If you're a strong safety and you line up to the tight-end side and that tight end motions across, you can't flip because you don't know if he's going to stop and come back and if he does you're looking bad. Guess what, if you have a 230-pound strong safety that's an in-the-box strong safety you can turn him into the free safety just motioning one guy across the formation. So it puts more (emphasis) on having multidimensional (guys).
The whole article is fascinating though, I urge everyone to give it a read. It's a good look into what DA and Tarver want to achieve with our defence.