For long running series like Naruto, One Piece, or Pretty Cure, ordinarily, 720p is enough. If it's a particularly beautiful show like Fairy Tail it's better to have the blu-rays, though. And when the current best option is just a measly 480p dvd, upgrading to 1080p blu-ray is worth it.
This is the case with Go! Go! The first 9 episodes are from a dvd, and the remainder of the series is in 720p. So a helpful internet fellow has provided the first 9 episodes in blu-ray subtitled brilliance, so we don't have to bother with 480p anymore, but can still be contented with the 720p section which really doesn't need an overhaul.
Now the only remaining low resolution Pretty Cure is from the movies and the first three seasons. This year has made a great deal of progress on the issue -- covering Yes, Go Go, Fresh and Smile. Maybe next year all of Precure will be in high definition. :).