Causation vs. correlation. Good teams aren't good because they sign their draft picks to second contracts, but good teams often:
1. Draft players worth keeping, and
2. Manage their resources to keep those players.
Lots of teams draft good players, but many struggle to keep them due to the cap. Good teams have to manage all of their resources.
If it was as easy as just signing guys to a second contract, we'd have re-signed Chad Jackson and Brandon Tate. If the Raiders re-signed JaMarcus Russell, it wouldn't make them a better team. So this is all about correlation.
As for the success rate of draft picks, I always go by old friend Herm Edwards. You play to win the game.
HELLO???
YOU PLAY. TO WIN. THE GAME.
Maximizing draft pick potential is not the point of the game. Draft picks are valuable resources, but they are just one form of resource. If we wanted to get the most out of our 2nd round pick last year, we would start Jimmy G. But if we are playing to win, we start Brady.
James White plays when he's the best option to play, not because we spent a draft pick on him. Aaron Dobson gets no advantage by being a 2nd-rounder, nor should he. The guys who can help us win today are the guys that play, draft position or potential be damned.
There is also the matter of what you are aiming for. I think BB realizes there's not a huge gap between the 4th and 7th rounds in terms of talent, but you often see him take some moon shots in the day 2 range. I think he's looking for 1st rounders who dropped for whatever reason instead of safer guys. In other words, I think he's going for a homerun instead of a single. And why not? If he thinks he can coach up some of the day 3 guys to a safe 2nd round level, why not aim big? People point out the misses, but the hits tend to be huge homerun like Gronk, who would be a top-5 pick in a re-draft.
The Dolphins and Bills and Jets usually get more out of their rookies, partly because they draft higher, partly because they get to play for the future, partly because hey are desperate and lack depth at a lot of spots. Kyle Wilson sucked, but got to play lots because they had no other options. E.J. Manuel and Geno Smith have way more starts than Jimmy G, but not because they are better players.
So draft picks are a valuable asset, but let's not overlook the true point. You play to win the game. And we've done that better than anyone else since BB started.