From what I understand though most/some road shoes are compatible with SPD cleats. I may be wrong though...
Some but not most and those which are treacherous; walking with a small metal cleat non reccessed is like walking on ice!
In response to the OP,
I have SPD-SL (Road Cleats) on the Bianchi and for me the slight advantage over basic SPD's is that the bigger plate provides for more comfort on longer journeys and slighlty more energy transfer (but that is mainly due to stiffer road shoes). The Major drawback however is walking off the bike.
On my other road bikes however, I use
A520 (touring pedals) which removes the smaller platform difference (plus IMO they look good on a road bike

) and wear a pair of stiffered soled Spesh BG MTB SPD Sport shoes which minimalises the power difference. The advantage of this system you can still walk.
I prefer the latter A520/ BG sports (SPD) combo tbh and use it nearly all the time on club runs or on solo rides and touch wood nobody has ever accused me of being slow
