Premium gatcha is useful, but you can get the same units from silver gatcha (although with lesser rank (up to SSR+ for story girls and only R for premium ones) and much lesser probability). Also, you can use plat tickets only for 1x summon, if you want to make 11x summon, you'll need [plat 11x ticket] (if I'm not mistaken, you'll get it after a certain progress in story missions). As for unit's strenght - it's something like: Premium girls > Event girls > Story girls >>> regular junk.