I see one huge upside to it, for me anyway. Ninja skills have every potential to hybrid perfectly with melee. You can perform all close combat skills with a shurikin, and based off the stats of my Starblade, I'm thinking a properly upgraded and reforged shurikin could be an effective melee weapon, meaning you can fully hybridize without switching weapons Also, I think my the ninja skills have the potential to blend well with melee combos, although I'll need to test that theory on so. Even monsters that are actually somewhat challenging.
As long as you never want to use normal attacks, anyway. That's the big difference between shurikens and melee weaps. Shuriken normal attack sucks a lot of ass. Also bear in mind you can't use FH with one.