i've used SPD's since the mid 90's... as i've branched out onto the road and higher miles I've began to understand where a road pedal would be advantageous... my MTB shoes are high end carbon sole shoes which is a good thing, but the more miles I do on the road the more I see the merit of a large platform from the pedal/cleat itself, I've been using shimano a520's of late and the platform seems to help with help with longer rides... based on my experience I'm planning to find some decent shoes and try out speedplays in the not to distant future part of that is so I can keep a consistent Q factor as speedplay will let me get longer spindles
i'm sure walking in them won't be great... but neither is walking in a GOOD pair of SPD shoes, if I cared about walking long distances I wouldn't go with clipless at all...