If the shoe sole is Rigid, as you walk your foot flexes. Your heel will try to ride up out of your shoe.
Do you want to restrain your foot or let it slip out like a penny loafer ?, you choose..
How walkable are the MTB shoes? because the shoes I wear on the bike I also want to be able to go hiking about 30km in.
Get double sided MTB pedals and ride in your Hiking Boots then .