I started out riding with flat pedals and flat shoes and a lot of people kept telling me how I would climb faster and feel more secure downhill with clipless pedals.
Then I participated in a one week course (MTB clinic style) where the trainer (who is very well known for training various world cup/olympic athletes) insisted that everyone used flat pedals. His reasoning was that using clipless pedals could cover up mistakes in riding technique and using flat pedals would result in "leraning it right from the beginning".

I never switched to clipless pedals after that. I feel more secure downhill, because I can jump off my bike faster or fall without thinking about unclipping. And by now I'm also one of the fastest climbers in our group, without "the more efficient pedal stroke".