Its hard to sum up, i think there is at least 4-5millions on exhanges with various holders, last time i looked at the block explorer 40% of the coins where top 100 address so less than 6millions-minous 4-5mill from exchanges  maybe and also currently i see 314 incentives nodes from various individuals thats 3.1millions in collateral.
Pretty much spread out at different levels out of the current 14.7mill. Next year only ~300k coins will be mined.

As for use case well its the fastest electronic cash available that can handle and scale to the mass.
Thin client is comming also, which allows you to use a wallet without downloading the blockchain to send and receive coins in a decentralized way, because the wallet is relaying to the incentive nodes.