One other advantage of the SPD pedals for a road bike is that there are quite a few pedals available that have an SPD mount on one face, and the other is a traditional flat pedal face, for use with regular shoes. That doesn't really matter on a bike that you will always ride fully geared up, but it is handy to be able to just hop on the bike with regular shoes on if I'm just on a short jaunt with my five-year-old.
(There may be similar pedals for look-compatible cleats, but I don't recall seeing them.)