I have similar pedals in my collection (nashbar/forte campus pedals). Idea sounds great in theory but in practice I didn't like fumbling to get to the side I want to use. So they are relegated to guest bike usage.

Shimano and others do make platform adapters (like these http://www.nashbar.com/bikes/SubCate..._200276_200316), so you can turn any SPD MTB pedals to one-sided platform, if you don't want to buy the PD-M324. One of the Gary Fisher hardtails I bought even came with them as standard accessory (on top of the PD-M505 pedals).