J1sx4:
Not sure what coins are supposed to be used for, other than paying to enter a match. I have over a million coins. It would be nice to be able to exchange some for credits.
That's about all they are good for, along with upgrading Apparel items. There are also a few club sets you can Rent with them, but to be honest those clubs aren't much better than the starter set given you at the onset, and the rental deals are horrible price-wise. Best to just save them up for when you get the opportunity to upgrade apparel to the next level or use them as entry to more coin games.
And as to exchanging for credits - that ain't ever happening. Coins have such a low value the exchange rate would be absolutely abysmal anyway.