Trail Jogger

I've searched the forum but need to ask the question in a slightly different manner because I've never ridden with cleats.

I can understand the need for cleats for road bikes because they generally ride in a smooth straight line but can't understand why one needs cleats when mountain biking. My current hardtail has stock Schwinn pedals that don't require cleats but I'll soon be buying a bike for which I'll also need to buy pedals and shoes separately. While cleats may help for speed and jumping, would the release be quick and easy enough to prevent spills?

Another way of asking my question is, if you're riding along a rock ledge where you might need to put your foot down to avoid falling over the edge, would you keep your foot in the cleat or just on top of the pedal?

Thanks in advance for your responses.