I think letting a leader emerge is fine under "safe" and/or training circumstances. I am even fine with pushing a new leader to emerge under those circumstances while holding myself back from taking the lead.

I recall that while in leadership school in the military, those of us with known leadership ability were now allowed to take charge of any situation, forcing those without to step up and learn how to be leaders.

However, the more technical or dangerous the situation, a clear leader needs to be designated or recognized.