As far as I can tell, the only reason is... he looks badass ;P

On the real tho, the chases of a lot of relatively popular ninjas seem to create low float (tobi, kank, kabuto, gai, kisame) and lots of ninjas in general chase from knockdown, so he's much more valuable as a summon than, say, Giant Katsuyu.

Finish an actual survival trial fight and you should get your rewards for the first 9 as well