I've used mtb SPDs for many years. I also have Look type road pedals. The SPDs are easier to clip in, and just as lightweight. You can get good lightweight road shoes with super stiff soles that take SPD cleats, but walking on the little steel cleats is problematic and really rough on floors. The big plastic Look cleats are easier to walk on, generally won't damage floors, but still are nothing you could walk any distance on. They also wear out pretty quickly from walking on pavement or concrete. Another big difference is that with hard-soled road shoes, you can't really put any force on the pedals without clipping in because your foot can very easily slip off the pedals. With mtb shoes you can tromp on the pedal and the rubber soles will help prevent your foot from slipping off the pedal. This is very useful when getting started quickly after stopping for cross traffic. A few strokes will get you quickly going across the road and then you can clip in.

With stiff soled shoes, the soles form the platform for the foot and I can detect no difference between the mtb pedals and wider Look pedals.

I feel mtb SPDs or Eggbeaters with good quality, light, stiff MTB or touring shoes are much more generally practical. The only thing you're sacrificing IMO is the somewhat greater weight of the shoes.