The usage of the coins are almost explained in this update

[DededeCloneChris]

Now we know the coins are for, for bet to players. But still, what REALLY you can do with the coins? Are they the same feature as the friend vouchers of Metroid Prime 3?

[JrDude]

probably to buy stickers and trophies, 'cause if they were for betting, that would be pointless, it's like betting nothing for nothing

[QingXin]

It's already been disconfirmed to be for a shop, so I'm guessing the Lottery... Again.

[Eizweir]

So all you would do is gamble with them?

[thunderhero4]

sounds like a boring idea to use coins for just gambling, and im sure since sakurai "thought of everything" that there will be pleanty more fun ways to use coins :)