Not that way. Stegen has the ambition to be the #1, Roman or Zieler do not. That's the difference.
He does not have the ambition to be the #1 in this years WC(Neuer's the clear cut #1) and it doesn't matter what Weidenfeller or Zieler have in plan for their future.
Again, what's bad about it? It's a classic thing to do before a big tournament. He strengthened the position of his best keeper. As much as Stegen deserved to go ahead of 3rd keeper, he would have added nothing to the team and that's what international coaches keep in mind. So why take him or Leno?
There's no need to strengthen Neuer's position.
He's the clear #1 in Germany's goal, the "world's best goalkeeper" and playing for Bayern Munich.
And if you're playing for Bayern and are pretty good then you're going to play for the NT. That's just how the politics in Germany work and why guys from other teams(even the Dortmund players) have a hard time playing.
Taking Leno or ter Stegen would be good because they're great young goalkeepers and experiencing such a tournament(even as #3) would be good for their future.
Though, with Löw you never know. He prefers guys from his region(Freiburg, Stuttgart, Hoffenheim) and doesn't seem to be quite as interested in guys playing for Gladbach.
I mean, he also didn't nominate Max Kruse, who's the 2nd best German scorer of the Bundesliga(behind Reus, with 12 goals and 12 assists) even though they had Kruse shoot promos for the NT on Tuesday and told him his medical would be on Thursday.
Are you bilingual? Because if you're not then I can see why you don't see the profit in this.
Well, I'm a German writing in english on these forums.
Though, we're talking about a week or so and that's not going to be a big difference given that he's learning the language for quite some time and will be in Spain for what hopefully be many years.