Some riders like to use both (SPDs for racing, Flats for playing) and it saves having 2 pairs of shoes.
Plus, if you're on SPDs, unclip and can't get your foot on in time, the 5.10 grippiness can save you for a few corners.

 okay that sounds rational.. except for the spd part

 you dont have to run them with SPDs anyway. most of 661s shoes have the option of spd or flats

 yeah 661.. but this is the first 5-10 one..
i thought 5-10 was the upstand against spd..
you can ride without spd.. as long as you've got 5-10's