I just went and saw these at a fish shop here in Hong Kong, they definitely look like spiders... I am not afraid of spiders, but I don't want that critter crawling in my tank. Cherry shrimp and amanos are much more attractive.
Now if the microcrabs were of a more attractive color, I would consider, but with the dull gray color, they look like spiders crawling in the tank. :p
I don't know if they NEED hiding places, but mine hides all the time lol. Some hide in the holes in my lava rock and one has taken residence in a shell permanently. I have couple of Cardinal Tetras in the tank with no problem. The crabs do tend to hold their ground by stick their legs up when a shrimp get near them.