If we draft between say 16th-18th is that really so much different than drafting say 10th-12th? How much a player ultimately contributes to the team seems like pretty much a crapshoot in such a small range.
The Saints missed out on Jordan Jefferson, Kenneth Murray, and Jalen Raegor( Picks 22,23,24)the year they took Cesar Ruiz. The year they terribly needed WR and LB help, and they had to settle for OL based on what was still available in their cloud.
So despite your assessment, a few slots can make a big difference in availability of players that fit your team at positions of potential higher need.