Please check Thread:Portal talk:Eu/Language fallback to Spanish (es).
Adding Spanish translation would make less Wikipedia in Basque less atractive, as people wants it in Basque and "understands" that some things are in English, but not in Spanish.
Do you have some statistics on how many people "understand" (the usage of) English but not Spanish?
The problem is not about statistics, is about the logic below this decission. Nearly all basque speakers know Spanish, as first, second or third language. But the main point of having a Basque Wikipedia is that is not in Spanish or in French. We can have a fallout in Spanish or French, but for regular user finding something in English is like "well, this is not translated" but finding it in Spanish would be like not having this project in Basque language, as he/she is reading EU:WP BECAUSE is en Basque.