Walking up hills is easy - unlike road bikes, mountain bike clipless pedals usually have the cleat recessed into the shoe, so in other words you are not standing on the cleat. i had some Answer shoes which had enormous lugs, they were great for running around in the woods. Lots of people wear clipless shoes for bike riding and just never install clipless pedals or cleats just because the firmer footbed provides your foot with the optimal position for pedaling.

As far as falling out is concerned, I agree with others - it usually isn't an issue. If you fall, you normally unclip automatically, especially as riding with the pedals becomes second nature. I have endoe'd before and unclipped and hopped over the handlebars - believe me, I was surprised when I did it!