Much more useful to the Germans would be using the Canaries - both for long range maritime patrol and refueling/rearming submarines (and allowing a little R&R for crews). However the RN is capable of striking this base with naval gunfire, or even a carrier strike without actually declaring war on Spain - simply stating they were attacking German forces. Of course once the US joins the war, these bases become completely indefensible and at that point Spain risks the USA/UK taking them over "temporarily" to prevent use by Germany. As much as Franco did not want to get in to a fight with Britain, at least until they were down for the count if even then, he REALLY did not want to be at war with the USA. The US could easily cut off the flow of oil, food, etc from the western hemisphere to Spain which Spain desperately needed. Absent these goods from the western hemisphere Spain would collapse economically and face famine.