Some reasons why XCP has value:

1) Trading with XCP on the distributed exchange is (and always will be) cheaper and faster than with BTC.

2) You can use only XCP for making bets, CFDs and asset callbacks. Forthcoming complex features and financial instruments will also likely be restricted to denomination in XCP.

3) All future proof-of-stake voting, e.g. for protocol changes, voting functionality, DACs, etc. will be determined by XCP holdings.

XCP is the native currency of Counterparty and its sole first-class citizen. If the Counterparty protocol has great value (as I'm sure that it does), then so does XCP.