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.
Thinking out loud here, but is there any chance of getting a BTC proxy token that is tradable. My ideal token would be composed of the amount of XCP that corresponds to a single BTC. This means I would effectively be trading XCP but I would not have to think about the value of XCP, I could still price everything in BTC.
I say this because as I see it one of the main issues right now with the decentralized exchange is the additional mental cost of calculating things in XCP as opposed to BTC.