If somebody is moving to Ibiza Spain, where the locals speak Catalan, what good will learning Castilian Spanish do me?

In Ibiza you will find lots of people who speak Spanish, not Catalán. Besides, remember that Ibiza is a small island and as soon as you move to other places in Spain you will need Spanish to get by.

Even in Catalan region, Castilian (What we commonly call "Spanish") is widely spoken and understood, especially in a popular tourist destination such as Ibiza.