each time in my riding history that I went from regular running shoes (very flexible, caused arch discomfort) to stiffer soled bike shoes (but still with leather clips) to early spd mtn bike type shoes (around 91 or 92) to much better and stiffer spd mtn bike shoes---each time my foot comfort increased, as well as efficiency. So for me it doesnt make sense to ride in a shoe that isnt very stiff and fits me really well (fit is not a factor here comparing biking or non biking shoes, more the stiffness).

sure, taking an extra pair of shoes takes up room, but I for one cannot wait to get out of my bike shoes, and into something else at the end of the day after a shower. Sandals are my choice, just to let the poor old toes to air out after being scrunched all day and pushing against a pedal or two.

The only time I could see non spd shoes as being prefereable would be if you knew you'd be walking a bike a lot, but even then I'd lean to stiff soled feet simply cuz of the riding factor of it being better--and considering the hours pedalling vs walking in those shoes, to me its an easy decision.