The twist is that those who join the war on an Alliance CB cannot take provinces in a peace process
So if you join your ally in his war, you can only
* release vassal
* release country
* reduce sphere
* revocate cores, or
* get gold
Or did I misread it?
I don't think you did, and that's really the spirit of a true call to arms anyways, isn't it...? Your impetus to joining the conflict is to help your neighbor, not attack the other country. As such, getting some gold out of the deal to pay for your efforts is rather reasonable, as is the demand the other nation remove cores/claims on you or your neighbor.
If anything, it looks like a great way to pass the time waiting for Infamy to cool a bit. And just think of all the cores you could strip to prevent nations from going where you didn't want them to go. It's a little back handed and sneaky, but what else are you going to do while you wait...?
T