Mountain bike shoes have tread around the cleat that make it much easier to walk than with road shoes. They can still be used with most road pedals, or you can put mountain bike pedals on a road bike. I've used Shimano A520 and M515 pedals with one set of mountain bike shoes and had no problem going back and forth. I'm going to get some M534 pedals next (they are SPD on one side, platform on the other) so I can wear any shoes I want whenever. If you race intensely, you may want the road shoes and pedals since they are the lightest, but for most of us, pedal and shoe weight issues are trivial.
Even with mountain bike shoes, the cleat may still make contact with the floor, so you won't be wearing them in the house much if you have tile or hardwood.