Bread and butter, sure. But it's definitely in orders of magnitude easier having all pilots with the same SOLI. One set of rules, one set of forms, one authority to deal with. Less complexity, smoother process.
Imagine a hypothetical airline which has 31 pilots. Which one would spend less resources (time = money) - the one where all pilots have licence issued by the same authority, or the one where every pilot has their licence issued by a different EASA member state (there are 31 of them)?