I think it's a mix.
My belief is that every player on the team apart from the unquestioned stars need to feel as though they have to keep working hard and producing in order to keep their spot. Unless we're talking about perennial All-Stars, nobody should feel as if they are guaranteed heavy minutes.
With that said, I also think that some young guys need to be given an opportunity play through their mistakes in order to fully develop and blossom into players capable of filling a large role on a team. Some guys have the mentality and the work ethic to go from 3rd stringer to star over 4-5 years playing behind accomplished vets who know how to play defense and take the right shots.
Other guys, I think, can come into the league, get buried on a depth chart, and never really figure out what their role in the league should be. Somebody like James Young, for example, who was probably not actually taught how to play team basketball in high school or college, really needed to go to a team that was willing to give him minutes and have patience with him. Build up his confidence and make him feel comfortable in a role but also hold him accountable.
I don't think all young players are going to flourish in a scenario where they feel like if they make mistakes or don't hit shots, they will be yanked quickly and relegated to the D-League.
I think NBA teams that are smart take the mental side of things into account when they draft. You're better off taking a less-talented guy with the makeup to succeed in the situation he's entering than drafting a guy with all the talent in the world but a poor disposition to succeed on a team that wants him to learn a certain system or bring him along slowly.
With Young, for example, I think the Celts probably drafted him expecting that they were going to be bad for at least another season. Instead, they returned to the playoffs, and Young has never really gotten the opportunity to try to fill a void in the team's rotation. His only hope has been beating out a more experienced or polished guy ahead of him, and I don't think he had much hope of being able to do that given how raw he was from the start.