I think abandoning Caralis is always a bad idea. No matter how worthless it might be, the Julii *want* it. As long as you hold Caralis, Julii will throw all their forces *there* and nowhere else, making it very easy to anticipate and thwart their attacks. Caralis is just a short boatride away from Carthage herself, so reinforcing it is not much of a problem.
If you abandon Caralis, then it opens up a whole front for the Julii: they will threaten Palma, Lilibaeum and even Carthage.

In short, Caralis might be worthless economically, but it is a real gem strategically.