For commuters, for light city riding, mountain pedals and shoes are great. But I find some people, lightweight people most especially sometimes miss some of the benefits of the wider SPD-SL or Look Keo pedals. It's not just that power transfer is more efficient — for most of us, that extra watt isn't a factor — but what DOES make a difference is the wider power transfer platform.

If you're wearing high-end, über-stiff shoes it may not matter much, but anything shy of that, if you're putting down major watts or major weight on the pedals, you may find that the wider platform offered by road-specific pedals affords you greater comfort, and can avoid the hotspot of soreness in the center of your foot which narrower pedals have been, at times, known to cause.

For walking around, Shimano's SPD-SL pedal covers are certainly good enough.

Oh — don't overlook the better cooling offered by road-specific shoes as well. A good pair of road shoes with road pedals is, when you're on the road, a wonderful interface to the bike.