Larger platform road pedal systems (Look-style) allow for more contact area--larger cleats. Generally, that means you can have more efficient power transfer to the pedals. You may be less likely to get hot spots. Hot spots can often be fixed by tweaking your cleat position, even for small MTB cleat systems, so I wouldn't worry too much about that. The stiffer shoe also helps with efficient power transfer and can help you ward off some foot fatigue. There are plenty of stiff MTB shoes, though. I would say that if the walkability of your shoe when riding the road bike isn't that much of a priority that you should just go with a road system. Some MTB shoes can be heavier too. For long hours on the bike, I'd prefer a lighter, stiffer road shoe that I could just forget about and focus on pedaling, but there's nothing wrong with using a MTB system.