I think it would be good to have PR reviews. I'm not a maintainer, only a developer, but I try to read most of the PRs and there are three main reasons for me to do it:
*) I learn a lot of Java, design patterns and coding practices by reading other peoples code.
*) I learn about how JMRI works. By reading Issues and PRs and what problems the PRs try to solve and how the PRs solve it, I learn more about how JMRI is structured.
*) I learn about there JMRI is heading. By reading PRs, I get a glimpse of what JMRI is going to be tomorrow.
Also, people grow if they get responsibility. If nobody trust me, I don't need to take responsibility. If people see me as a leader, I need to step up to show that I deserve the trust. There may be someone that can't handle the responsibility and there the project leader needs to remove the trust, but for most people it works fine.
One really important factor is tone, how we treat one another. A "thank you" or an excuse means a lot. I have sometimes failed to have a good tone, but I try to apologize when it happens. The absolute most important factor to why I'm still active as a JMRI developer is the encouragement Bob J has given me from time to time. Harsh words leads to the opposite.
I have done several comments on PRs. Those comments are almost never requests, but thoughts. Sometimes I'm right, sometimes I'm wrong. But I have learned a lot from the responses I have got to my comments on the PRs.