You won't destroy your cleats if you walk around in them but it can definitely add to the wear and tear. If you think you'll stop'n'shop a lot in your cleats and you don't want to carry around shoes, pick up some cleat covers. Just keep them in your saddle bag and pop them on to protect your cleats while you walk around.

I think that clipping out eventually becomes second nature although you can still have the occasional emergency when you just don't clip out fast enough. A couple of months ago, I had to stop very suddenly when a car pulled out in front of me and, you guessed it, I didn't clip out in time. The driver saw me and stopped so I would have been okay to keep going. Instead, I lost my balance and... timberrrr. The bike was okay and I only scraped my knee. Bu much better than a possible collision with a car!