The reason you get SPD shoes ( and pedals, of course ) instead of SPD-SL or Look or whatever, is because you want to walk in them.

You won't wear the cleats down, because they're recessed, unless you're walking on very uneven, rocky surfaces. Otherwise, they don't make contact with the ground. Also, the cleats are $20, and last a few years, so even if you do wear them down, it isn't the end of the world.

On some surfaces, like freshly cleaned smooth tile, the shoes can be a little slippery, probably because the contact area with the ground is small - those ridges along the edge that surround the cleat. I saw a cyclist slip and fall once in a restaurant in bike shoes.

I wear my SPD/MTB shoes when I bike to the store, and to and from my desk at work. I keep a pair of old hiking shoes under my desk, although when I take a late lunch I sometimes don't bother getting out of the bike shoes.