I always thought that this was a bad move on Pius' part. After all, the action confirmed in Englishmen's minds that the Pope was primarily a political manipulator and not a spiritual leader. Also, it led to worse persecution of the recusant population in England.
However, I recently learned that Elizabeth was not truly a rightful heir by both English and Roman standards. Either way, she was technically an illegitimate child of Henry VIII. If King Henry VIII did have a sacramental marriage with Catherine of Aragon, then obviously Elizabeth was illegitimate. Elizabeth's mother Anne Boleyn could not have held a valid marriage with King Henry VIII because Henry had for quite some time had sexual relations with Anne's sister and thus the laws of consanguinity and affinity prevented a union between Henry VIII and Anne Boleyn. Thus, any child of Henry VIII and Anne would have been illegitimate.
Also, apparently Elizabeth's coronation had been fully Roman and thus she had solemnly vowed to uphold the Roman Faith. I might be incorrect on this point and I'd love a correction if I'm wrong. But if that was the case, her imprimatur of the 1559 BCP revealed her as a royal perjuror.
What do you think? Did Pope Pius V do the right thing at the right time?